Mercy Thompson/WMG

Mercy can somehow use her magic to keep people alive and well.
In Moon Called, when Adam is trying to stay conscious in the bad guy's lair, she says, "It felt like he was using my eyes to pull himself up from wherever the drugs had forced him. I tried not to blink and break the spell." In Blood Bound, when Warren has been flayed alive, she says, "I couldn't take my eyes off of Warren. I had the strangest feeling that if I quit watching him, he would die." We've seen Mercy use her magic to "catch" a death spell meant for Adam. The vampires must have further reasons to fear walkers than just their ability to see ghosts and sense magic at work. What else is there about walker magic that Mercy hasn't figured out yet?
 * I assume its not really a power to keep people alive so much as speaking directly to their soul, much like how she talks to ghosts.

After Mercy completes the mating bond with Adam, she will gain a werewolf's ability to heal.
In Cry Wolf we learned that mates share magics through their bonds, and sometimes gain new abilities entirely, such as knowing where the other is or what the other is feeling. The obvious magic for a non-werewolf to gain from the mating bond is the werewolf magics of their mate. Adam will gain new abilities too, such as sensing other people's magics like Mercy can.
 * As of River Marked, this seems to be Jossed since the mating bond is complete but she still gets beaten up quite a bit.

One of the books will feature skinwalkers as the villain.
In every book, Mercy mentions them when describing walkers. For a monster that hasn't shown up, we know quite a bit about them. They will show up eventually, perhaps in the last book.7
 * Almost certainly true on the grounds that a skinwalker shows up in just about every urban fantasy series ever. They're like the hot Exclusively Evil creature of the past few years.
 * The Author notes for the next book, River Marked, have said that the Author intends to "work in more walkers and the Native American culture".
 * River Marked has more walkers but no skinwalkers yet.

Mercy's father is Coyote from Native American mythology.
To elaborate, Mercy only knows her father as "Old Man Coyote".

Bran was Sir Marrok.
He's old enough to have inspired the legend of Grendel. At that point, the couple extra centuries to put him in King Arthur's time makes little difference.
 * I like it.
 * CONFIRMED! Hunting Ground confirms that Bran is the original Marrok.
 * Not like it was any surprise to those paying attention in Cry Wolf.

Mercy's father was killed by vampires.
He died in a car crash when a piece of metal cut his neck and he bled to death before the ambulance arrived. Or so they say. Given vampires' determined efforts to extinguish walkers, it becomes more plausible that he was murdered by vampires, and the metal from the car crash was introduced later to cover up the bite marks.
 * Yeah, this is pretty much implied in the series. I mean, his neck was cut by something sharp, resulting in him bleeding to death, and he has vampire enemies. Hrmmmmm, I think he was killed by a truck accident, don't you?

Warren is planning to turn Kyle into a werewolf.
Thus him (Warren) having lots of raw meat in his truck during the fifth book, Silver Borne. Remember? It was an interesting bit of Foreshadowing, especially as Warren refuses to say why he has coolers of raw meat in his car. And does he not also have camping gear? Presumably it's so that when Kyle's being changed they can go out to the country so no one will be disturbed.
 * It does make sense, and these details are among the ones that haven’t led to nowhere in the books yet.