I would say yes. This would be my opinion/perspective on the idea of a BIG crossover movie. I actually already posted in the Avengers vs. X-men thread in The Avengers sequel forum but I feel this thread is perhaps more appropriate.
Here's some interesting feedback on this idea. Several months ago, right around the time Thor:TDW was released on DVD and Blu ray, I decided to get the stand alone DVD of the first Thor film since I only had the 3D Blu-ray combo that had the bare-bare bones DVD. Only place in my area that actually had a copy of the Thor DVD in stock was Toys R Us. So at the checkout the cashier asks me if I saw the new Thor film and what I thought, I said it was all right, I enjoyed it but I was really looking forward to the new Captain America Winter Soldier movie, they replied that it looked cool but what he was really waiting for the most is the Avengers/X-men team up. Now that there is something. The cashier from my perspective was the very definition of the GA, general audience. I replied that's a cool idea, maybe something like "Secret Wars" or "Onslaught" but he wasn't really familiar with the actual comic book arcs. What that shows to me is that there is a demand among the general audience(non comic fans) for an X-men/Avengers team-up crossover flick.
My opinion, if Marvel/Disney/Fox/Sony were to work things out and pull off a crossover epic, I can almost guarantee if it's well made, it'll gross something like $2.7 billion WW easy. It would be one for the record books. Mark my words if they ever pull this off we're looking at a gross north of $2 billion WW. This is something if all parties were to set aside egos and actually work together and work out all the rights and who gets what and what not, they could pull off something extra special in cinema history. They could use some comic booky style plot devices to explain a possible crossover. "Secret Wars" and the Beyonder should be used as a possible template, the Avenger's MCU and the Fox's X-men-verse would be 2 totally separate alternate worlds/universe/dimension and the Beyonder or some similar entity could exist is a place beyond time/space, a nexus of all realities and it puts a pause on the two worlds and spirits away groups of the top heroes and villains from both worlds to compete in contest that if lost would have catastrophic consequences for both or all worlds but if won things would return as if nothing had happened, not unlike the SW arc from the comics.
That's one possible way and if they ever did pull something like that off once then maybe I could see them using something like "Heroes Reborn" or "House of M" as possible "sequel crossovers". The whole multiple worlds/realities, they could get really creative with that. For the most part it shouldn't effect the overall continuities for the separate universes and essentially be more of a "What If" novelty kinda thing. I know it's probably very unlikely but it's fun to speculate. Even if they were to get this together I would say maybe not until something like phase 4 or 5 of the MCU or when the first phase cast decide to start moving on. A lot of people I know first became Marvel fans not from the comics or movies, or even just the animated shows on Fox, but the video games Marvel/X-men vs. Street Fighter/Capcom. That was a big, big deal for a lot of people I know from way back in the day. If or when they make a crossover epic, people will show up.
Honestly I'm all right with Fox having X-Men, DoFP was pretty good although I still don't like how they cut their films for theatrical release and all the multiple Blu-ray exclusive cuts etc. I guess I'm all right with the X-Men being their own separate world/universe from the greater Avenger's MCU and web/TV series. Spidey I could see a case being made to find someway to get him in the MCU at some point, although I'm 50/50 on how Sony's been handling things. Thinking back now I would say ASM-2 was just okay since some time has passed and I've really had a chance to process that film. Hopefully things get better for ASM-3.
My ideal roster:
