Codex Alera/WMG

The Marat are partially descended from primitive LOTR elves, e.g. The Avorren, or a group who had just awakened and got sucked into Carna

 * Elves are shown to have close ties to animals, and if this is extended to it's logical conclusion, capable of bonding with them. Also, they can breed with ordinary humans, have sharper senses and ubiquitous white hair. Mind, they would be the same to LOTR elves by the time of the books as Buffy vampires are to original vampires: inbred country cousins.

The Canim, thanadents, and numerous other creatures native to the world of Carna are descendants of various prehistoric creatures from our world.
Word of God has already established that several creatures in Carna, such as the gargant, herdbane, Marat, and leviathan are all descended from prehistoric Earth creatures that got sucked into Another Dimension and became established there (ground sloths, terror birds, Neanderthals, and plesiosaurs, respectively), much as the Alerans did. It is quite possible that other creatures in the series that are not immediately recognizable as modern Earth creatures could have a similar ancestry. The thanadent is described as having feathers (as worn by one of the Marat), as well as sharp teeth. This information would seem to suggest it is a theropod dinosaur, more specifically either a basal coelurosaur, a tyrannosauroid, or perhaps some sort of maniraptoriform. In-novel, the Canes are described as sort of like a wolf, but at the same time rather unlike a wolf. A possible ancestor can be seen in the prescence of Tavars, a species of predatory animal that is described as both wolf and weasel-like, which seems to suggest they are descended from amphicyonids, or bear-dogs, animals that are oftentimes described in the exact same terms. The fact that Tavars are highly intelligent and native to Canea suggests that Tavars and Canim share a common ancestor, sort of like how humans and the great apes do. It is also possible that slives could have an origin in our world, possibly descendants of one of the Cretaceous species of venomous monitor lizard. The Vord, though, are most likely Starfish Aliens who fell through from their homeworld, if not from another Another Dimension entirely.
 * In Princep's Fury, it's all but stated that "grass lions" are really sabertooths.

One of the primary differences between Carna and our world is the fact that energy can be transferred more easily between objects in that dimension.
All of the major sentient races in Carna have powers which are not present via their counterparts on Earth (if the Alerans are any indication). These powers are all very similar, and oftentimes involve transferring their powers to one another. Alerans get their powers from their furies, which are sentient embodiments of the elements who give power to them. Marat get power from their chala, who are living creatures which give them power. Canim blood magic seems to draw power from other members of their own kind. Heck, even the Icemen could be considered as falling under this, as their telepathy could be due to Icemen sharing energy with one another for communicative purposes. All of this could be explained by Carnia having different physical rules than our universe, specifically allowing for the easy transfer of energy from one source to another. This allows organisms to share energy, and utilize new powers and methods of survival inaccessible to creatures that live in our world.
 * And the Vord's instant-communication through the troops by the Queen.

A group of Alerans will eventually fall through a portal to Earth.
After all, if things can come to Carna, who's to say that things don't come from Carna? This could explain the multiple instances of the Marat fighting the Vord; it's a battle that taken place across the different dimensions that groups of Marat and types of Vord have inhabited over thousands of years. One would wonder if an Aleran's furycrafting would work on Earth.
 * Better question: if it did work, would they be able to teach it to humans in this world?

The reason the Vord are a Captain Ersatz version of the Zerg is because they are the Zerg
The fact that they have drones and zerglings is not coincidental. They came to Carna through a portal somewhere in the Starcraft-verse. It was either a relatively isolated group or the Marat destroyed most of their technology, though.
 * Even if one discounts the differences between the Vord and the Starcraft 1 era Zerg, they're basically identical to the Starcraft 2 Zerg, right down to the new way Queens function.

The Vord are Tyranids
The current Queen is a particularly powerful and independent Synapse Creature who was thrown out of a hive fleet for being too interested in her food.

This series will get Canon Welding with The Dresden Files
Not soon but years down the line when Butcher has run out of ideas.

This series and The Dresden Files will cross over more traditionally.
After all, there has to be something beyond the outer gates. Fanon already says it's other planes.
 * You know how Dresden says that reaches of the Never Never just get weirder and more dangerous as you head out from Fairy? The Never Never is really really big, the world could well be hidden in there somewhere. Probably the result of a god's hibernation dream or something, which explains the lack of traditional magic and why it's undisturbed.

Gaius Sextus and Alera were more than just partners.
Seriously, Gaius was married to a beautiful young woman but showed her next to no attention, not even to try to produce heirs. That makes no sense until we realize that he had the power to summon an inhumanly beautiful, intelligent, witty female fury into existence whenever he wanted.
 * Building on this....

Septimus was the son of Gaius Sextus and Alera
Note that the characters remark how utterly diffirent Septimus was when compared with Gaius. Also, Alera's last words to Tavi are that she - the discrete entity that is Alera - will be with Tavi and his children forever. What if she means that they carry part of her because she was Septimus' mother? It would explain how utterly badass both Septimus and Tavi are, if they've got a bit of Alera itself running through them....
 * Fridge Brilliance as far as I'm concerned...

Aleran crows are clairvoyant.
Otherwise, why would they show up before battle is even joined? They're not following the armies, since they don't seem to be in evidence during normal troop movements.
 * They can also apparently see in the dark. In Real Life, crows are strictly diurnal.
 * This theory fits nicely with the one about Carnian animals (including humans) manipulating energy in ways that their Earth counterparts can't.
 * Their diurnal nature aside, in Real Life crows do in fact follow groups of armed men in war zones.
 * Or maybe the soldiers just don't notice the crows except for before battles.

Gaius Primus is Gaius Julius Ceasar or some relative of his.
In the prologue for First Lord's Fury Varg is reading a book by a man named Julius who is the commander of the legion that ends up stranded on Carna.
 * Gaius Primus definitely wasn't Julius Caesar, seeing as he didn't establish Alera until 1000 years after the Lost Legion's arrival. Besides, Julius was already long dead and buried when the Ninth Legion marched for Britain. It's possible that one of the Ninth's legionares was a descendant of Caesar, but not bloody likely.

On the opposite side of Carnia from Alera and the Canim continent is the Avatar continents.
It makes sense; bending and furycrafting are simply two different aspects of manipulating elements. The Avatar world simply hasn't had anyone learn how to bend plants because they haven't found a spirit/animal that can do so. Of course, this does mean that sooner or later, the Avatar world is going to have to deal with the Vord.....
 * Who Would Win: The Avatar or Gaius Octavian?
 * Quite obviously, they would team up. Together, They Fight Crime.
 * Powerwise, Aang beats even Sextus: Alera herself is like a weaker version of the Avatar Spirit that takes a more direct role in the world. He might even have been able to match the Vord Queen in the Avatar State. As far as strategy goes though, I think Octavian needs to take that one.
 * It's really the other way around, actually. In raw power, the Alerans are far stronger than Avatar characters in most regards, excepting watercrafting vs. waterbending. Avatar characters, however, have more versatility and endurance than Aleran characters. The average High Lord is an even match for the Avatar, and is likely stronger (Bullet Time, plus metalcrafting means they'd likely cut the Avatar to ribbons in close combat).
 * The Avatar world does have plantbending/crafting, it's just that so far it's just one guy in a swamp. Almost any Badass Normal might be an unwitting plant- or metalcrafter who doesn't realize they can do the kinds of flashy things that require a manifested fury.
 * That does explain how everyone is some degree of Made of Iron, and the Required Secondary Powers.

A sequel series set 100 years after First Lord's Fury will involve Giant Mechas fighting the Vord.
It's a hundred years after merit based furycrafting is introduced, which means a few generations of Tavi-like people have been running around creating a techno-crafting revolution. The technology of Alera will skyrocket far faster than on Earth thanks to the merit based furycrafting and the threat of Vord invasion. The furycrafting will allow the Mechas to work despite the Square Cube Law, and in preparation for the Vord in Canea the alerans will load the Mechas with every type of weapon available. And it will be awesome.

Carna is a popular dueling ground for Planeswalkers, perhaps even a tournament.
Each race is a Planeswalker's preferred deck theme. Those wiped out could be loses in the tournament. Building from this,
 * Are the Alerans a Blue deck or a Rainbow deck? The Vord are pretty clearly Green, possibly Green/Black... Also, the Marat are pretty obviously White.
 * I had the same thought about the colors of Magic...I would say that the Alerans are White (Well-meaning and orderly but not always "good," able to use lightsabers and The Power of the Sun. The most powerful firecrafting attack, a white-hot sphere that vaporizes everything inside and sears everything outside for a wide radius sounds and looks quite a bit like Wrath of God.), the Icemen are Blue (telepathy and control of air/water), the Marat are Green (closest to nature, most likely to not interfere in the life cycle), the Canim are Red (angry and warlike but disciplined, control over fire and lightning with their blood sorcery) and the Vord are black (only concerned about its own survival, willing to make any sacrifice or commit any atrocity to win, having a Zombie Apocalypse schtick).
 * I was looking less at elemental concerns. I don't like to think of M:tG colors as elements, but as strategic styles. Marat are large groups of relatively less-powerful fighters that work well in large groups, the Alerans seem to have either the Blue (ie screwing your opponent with devious cunning at every turn) or Rainbow (Taking bits of all and trying to use them synergistically. The Vord are Green because Green is the master of "Fast yet puny peons now, big huge evil monsters later." Possibly with black because of their propensity for self-sacrifice, as you mentioned.
 * I see the Alerans as blue-white. They're blue because they're scholarly and they have the most versatile magic system. Their whiteness comes partly from their organization and a good deal from Alera's I-help-everyone-equally-but-one-side-benefits-from-it-more shtick. The Canim would be red and white. Red because their magic is used almost entirely for killing things and because they're warlike even by Carnia's standards, white because of the caste system, especially when it restricts Varg from sharing information with Tavi.
 * Obviously the Vord are the Slivers, I think a mix that's mostly green and red since they mix cheap self sacrificing minions in enless swarms with raw physical power. Throw in some blue control spells for the Vord Queen. The Alerans are a white - blue SOCIETY, but the Aleran Military is White-Red: their tactical doctrine is based on mutual support, healing, and setting up for devestating fire blasts. The archery effects of woodcrafting and healing of watercrafting all fold neatly into white, while the fire blasts are obvious. The Marat are Red-Green like the vord, the classic barbarian colors, but with an emphasis on quality over shear quantity. Canin are red-white like the Alarans, but with more focus on individual creatures than the Aleran side's support emphasis. Their spells are offensive and very good at damaging fliers even if they can heal when neccessary, while red also encompasses their ferocity. Through in alot of high-attack first strikers.

Tavars are actually honey badgers
They're just larger and have adapted to colder weather. The resemblance is uncanny.

The Vord copied their telepathy from the Marat
During the first appearance of the Vord they display little to no furycrafting capability relying on physical power and Hive Mindiness. When they next return in Priceps Fury they begin to make greater use of furycraft and by the final book the primary Vord Queen has become the most powerful crafter in the series. The Vord in Canea however show no such ability because they were born after the Vord Queen fled Alera (and so have never been in contact with furycrafters before ). We know that the Vord have encountered the Marat on at least one occasion in the distant past so it's no great stretch of the imagination to assume that they 'copied' their abilites, indeed the comparison between the Vord and the chala bonds of the Marat is directly disccused.
 * The biggest problem with that seems to be that the Vord's telepathic powers are far greater than the Marat's- we have no indication that any Marat is capable of anything more than forming an empathic bond with their chala, while the Vord Queens not only maintain full telepathic control of their "children" (or at least, a few thousand in the immediate vicinity) but they also have completely unrelated abilities like mind-reading and illusion crafting. Also, the Vord's telepathy is so integral to how they operate that it's hard to imagine they could have ever not had it. I'd guess that if they did take anything from the Marat, they only strengthened what they already had.
 * I actually like this idea. The Vord were probably always a hive with the ability to adapt, but before the queen had to give orders like anyone else. Then they copied the Marat telepathy, developed a true Hive Mind, and developed conscious control over it to the point that they can create temporary bonds with creatures to read their minds and create illusions. It's even stated that Vord mindreading is reciprocal if you take advantage of the opportunity. It fits with the Vord Queen becoming the greatest furycrafter, just as the vord had become the greatest telepath in the past.

The Marat inadvertently created the Vord, or at least Takers
The key here is Doroga's description of first contact. The Marat met "A creature", and the description of events is heavily intermingled with references to Marat bonding. Furthermore, the original Vord apparently hijacked Marat women as incubators, and nothing similar occurs during the Vord invasion of Alera, nor is it mentioned in Canea. So, here's the theory: 1. A Marat attempts to bond with a Vord queen (probably it either looked sort of like the Aleran queen prior to getting blood samples or like a Vord beetle warrior) 2. The queen destroys the Marat's mind, and gets Marat traits 3. The queen also obtains Marat blood, and grows into a Marat-like form 4. The queen manages to produce Takers using the blood as a base 5. Taken Marat provide the first generation version of the hives that provide the Vord swarms during the Aleran war 6. The Vord obliterate the Marat 7. The Vord become a Horde of Alien Locusts, and finalize the system they use during the series at some point after the last war with the Marat (hence why the Wax Forest was left alone; the Marat would never have missed such a heavily infested zone intact if they'd encountered it before)