the concept of a team of mixed Avengers and X-Men is interesting
The Avengers title has been borrowing X-men characters on and off since the 1960s: Scarlet Witch, Quicksilver, the Beast, Wolverine, Storm, were/are all Avengers at one point or another.
The more "interesting" concept would seem to be the idea that the X-men might no longer be a team of "hated and feared mutants," but merely another super-team like the Defenders, the Champions and/or the Avengers themselves. If that's the direction, it would seem they've pretty much either said that mutants are now fully assimilated into the Marvel U and/or that there is no longer anything unique about the team's concept.
I might have to buy this just because of Cassaday,
That promo art looks really rushed. It took me seeing his initials on it to realize it was Cassaday. Is he doing full art or only layouts?