Kim
I feel like she's probably the biggest wild card on the show, one of the most speculated about. I've heard theories that involve her eventual death, or going on the run. I've considered those too, and those would make for a dramatic story, but I feel like that's less likely. I feel like the bigger it is, the harder it is to believe it was never mentioned somehow in Breaking Bad, narratively speaking.
I feel like it'll be something smaller, quieter, in a way more obvious. I think she'll probably just decide Jimmy was a mistake, and go back to HHM, and eventually end up with Howard, and Jimmy decides to never speak of them again. I feel like it would fit the show. It'd be a challenge to make it work for the character, but it could be done. Plus, she's not really done anything wrong, nothing worse than putting her faith in Jimmy.