Influenza mutates constantly, which is why it is hold hard to control.
It constantly changes its DNA when in contact with so many people and animals, and thus every year a new vaccine is created to try to catch up to it (and almost always fails).
I don't think viruses generally fuse with one another, but they can evolve and change enough to cause catastrophic effects. However, I think influenza could exchange genetic material enough to take aspects of another virus, I don't know how much. I could be totally wrong on this though, anyone who is studying medicine clearly feel free to correct me.
Ebola, for instance, could potentially become airborne one day - and should that happen, we're in trouble.
Rabies has already become airborne, several strains of it exist. The human version of Mad Cow could potentially mutate, but you know, it's a hard call.