I'd like an X-Men and Fantastic Four team-up, as long as it had a good solid director. Singer might do well with this, to make it realistic and atmospheric, perhaps with Spielberg as executive producer to ensure exciting action. I'd like a darker, more serious take on the FF.
The enemy? Doom and Magneto teamed up.
The story - Inhumans discovered when Crystal ventures into outside world after hearing of other evolved beings out there (FF and X-Men), with Doom and Magneto (with henchmen Juggernaut, Pyro, and one or two others) trying to take their terrigen mists and advanced technology.
X-Men and FF go to Inhumans, who at first attack them, before they all team up against Magneto and Doom.
Storm vs Crystal, Juggernaut vs Thing, and other showdowns.
Would need wise choices of which X-Men and Inhumans to feature and match against each other for fighting.