I don't think these characters are so much becoming Avengers as this is a X-men/ Avengers split team. That is like saying that Cap is becoming an X-man. This is a joint team not one or the other.
I dont think that this is going to be a happy relationship, especially after AvX, and believe that the team is going to have to be Avengers OR X-Men with their respective agenda being pursued (with the title suggesting the former more than the latter).
It could involve one bunch working with the other in some kind of redemptive attempt as it has been said that there will be a definitive 'winner' in AvX so it may be that one group or the other will be proven to have been 'wrong', bit I doubt it will be a case of mutants and humans playing happy families and working together for a brighter future.
I'm still sticking to my ' cool flying robot lion' theory for the time being as I cant recall the last time a large Marvel event put substance before style, cheap thrills or big Hollywood ideas.