I think Mike will be back in some way, we haven't seen the end of him. Walt killed his boss, and he was never a big fan of Walt to begin with.
I think we'll get to see where Gus came from in some flashbacks and learn why Don Eladio let him live all that time. In the original flashback scene, where Gus' "brother" was killed, we learned that something in Gus' past makes him a pretty important guy. I have a feeling there will be some people who aren't too happy he was killed.
If Jesse is going to do anything, he's going to go right for Walt. I don't see him going for Walt's family.
I hope this is true, an entire season with just walt v jesse wouldn't be fun, i really like when they include some high up drug dealers too.