Mario Kart/WMG

So, what's the deal with Go-Karting with Bowser?
It's all that crazy Mushroom Kingdom Royal Family politics, man. Truth is, Bowser's species came to Mario World millions of years ago on an asteroid named Bunuri, and they were the dominant species on the planet. But Mario World changed from a lizardy, cold-blooded place into the beautiful fungus-covered paradise it is today. The Koopas retreated beneath the surface, except for a few of their leaders, who interbred with humans in order to control the population.

These half-Koopa, half-mushroom people became the Royal Family, and continued to breed with the Koopa leaders until they all died out, because they were all intermarried with the Mushroom people. But something happened with the Koopas beneath the surface, and they became the violent, militaristic people they are today, and Bowser, the son of some nothing noble of a minor Koopa Kolony, overthrew his cousins' family and instated his own Military Junta, and proceeded to try to take the surface world.

The reason Bowser wants to kidnap Peach and Daisy, is because if he marries one of them, his children will be in line for the Mushroom Kingdom throne.

As to why it keeps happening, well, here's the thing... Bowser's got an inside man, who is part mushroom, part reptile. He's immensely trusted by the royals, and is such a deep cover agent that he plays himself as a huge coward and an idiot, in order to hide the fact that his allegiance is purely Koopa. But his name gives it all away: Why do you think he's called "Toad"?
 * Toads are amphibians...
 * And I do believe that the various Mushroom hat wearing people are given variations of the name "Toad". (Toadsworth being one example.) If one person can be considered a inside man because of the name, then what keeps the other people from being the same?
 * I'd always figured it was because Mario is a very poor judge of character and just thinks Bowser is his worst best friend.

The games are a test to see how far we can go before we lose it.
This explains why the cheating is so bad. Nintendo wants to see how far we can go before we lose our sanity due to cheating bastard AI. The game doesnt even try to hide it that well on some versions of mario kart. On 50 Cheating Capacity, your more likely to make it through the whole game. On 100CC, your gonna need antidepressants. on 150CC, if you get through this alive, you need a mental hospital. This also explains the rubberband AI and how the CPUs coordinate attacks on only you.
 * Or maybe it's to see how far the player can push themselves to beat the worst opponents out there, almost like a training montage.

Dry Bones in Mario Kart DS is the Koopa from Super Mario Kart and Double Dash.
In between Double Dash and DS, he died and was reanimated as a Dry Bones. However, he still had a passion for kart racing, so he came back. The Koopa in Wii is his relative.
 * Better idea The Koopa of Super Mario Kart loved karting immensely but when Wario stole and redesigned his kart (this is part isn't WMG,it actually says in the Mario Kart 64 manual Wario stole the kart),he got depressed and and killed himself after he wasn't allowed to enter the Super Circuit. Unfortunately he was granted permission for Double Dash since the Pipe-frame Karts were thrown out,so a relative took over for Double Dash. The relative needed a break for a while,and the old Koopa was resurrected and therefore was ready for DS. In Wii the relative was done taking a break and Dry Bones just had to come back again.

Rainbow Road is the cause of the Kart Races
Every few years, it pops up in the general vicinity of space in a new form. Whether its a sentient deity or the Mushroom Kingdom people just choose too, a Kart racing tournament is held in its honor, the last race taking place on the road itself.

Neo Bowser/Koopa City takes place in a Bad Future where Bowser succeeds in conquering the Mushroom Kingdom.
It could even be Mushroom City, renamed and rebuilt in Bowser's honor.

Mario Kart 7 will get Downloadable Content.
DLC for 3DS games has been confirmed as a feature.

Nintendo is aware of the many cries of Ruined FOREVER over the lack of some fan-favorite characters, possibly because they wanted to bring more attention to the four newcomers. DLC in the form of veteran racers remedies this. Don't think Nintendo forgot about those guys, either; they'll be playable in upcoming entries in the spin-off titles (Mario Party 9, Fortune Street).

Are you disappointed with the current roster? Were you clamoring to play as Waluigi, King Boo, and Dry Bones? There's still hope. As an added bonus, DLC for 3DS games will be free of charge. In the end, everyone wins.

After all, why have Waluigi Pinball if the purple guy himself can't race on his own track? The late November 3DS system update will add the ability for developers to release downloadable content for games in the 3DS eShop, but Nintendo president Satoru Iwata has revealed there'll be no Nintendo DLC until 2012.
 * Jossed. We'll be getting DLC for future games, though.
 * OBJECTION! Nintendo never specically said that old titles wouldn't get DLC support. The articles author said:

Speaking to investors after the semi-annual financial report, Iwata said:

Nintendo will also offer something like this for the titles Nintendo publishes next year, in a way that should be appreciated by our consumers.

That means we most likely won't see any extra cars or courses in Mario Kart 7 for example, but we could see additional stages offered in Luigi's Mansion 2.

No actual quote from Nintendo. Plus the author said "most likely won't". Emphasis on "most".

Metal Mario is Bowser's answer to Metal Sonic.

 * Like Eggman, Bowser decided to make an Evil Knockoff of the main character. Metal Mario in Mario Kart 7 is Bowser's new Dragon.


 * By extension, Metal Mario is an Early-Bird Cameo for his appearance in the next mainline game, as he has never appeared in a Mario game since Dr. Mario 64. It helps he's often depicted in spin-offs as Mario's rival.

The entire Mario Kart franchise is an epilogue to the Super Mario series.

 * After the countless conflicts between the Mushroom Kingdom and the Koopa Troop, as chronicled in the plaforming games. the two sides made a treaty that guaranteed the coexistance of the two parties. As a celebration of the peace, a karting tournament, similar to the Olympics, was held. The leaders of the two factions agreed to compete themselves, along with the Mario, two athletes from each faction, and two foreigners, Yoshi and Donkey Kong Jr. The tournament was held throughout both countries as a sign of unity. Even Bowser's mighty fortresses now became a sign of unity and cooperation. The entire affair was a success, and future tournaments would be held every few years.
 * Following tournaments expanded even further, including important dignitaries from a variety of nations. Notable examples include Donkey Kong from Donkey Kong Island, Princess Daisy from Sarasaland, Rosalina from Comet Observatory, and Queen Bee from Honeyhive Kingdom. As this occurred, locations from their kingdoms were also modified into race tracks. Eventually, old tracks were reused as parts of new tournaments.