That said, there are many things that would be different for this version of World War Hulk so that's not to say it would follow the same path as previous films. It could be Marvel thinking Planet Hulk would make an interesting and unique Hulk movie (and they wouldn't be wrong.)
Also, that's not to say that the entirety of Avengers 3 would be World War Hulk, just that he may feature in it. For instance, Avengers vs Ultron or someone and then uh oh, Hulk shows up... Or hell, like the comics, Secret Invasion is taking place and Hulk shows up.
I dunno, as much as I dislike the idea of running with a Planet Hulk (mainly because I never liked the cold, calculating side of people like Xavier and Stark) I still have faith in Marvel. Avengers 3 is still like what, 8 years away? I have enough faith that if that IS what they wanted to do, it wouldn't be out of left field and they'd lead up to it in time.
For instance, you know what would work? If they combined World War Hulk and Civil War. Have Tony and Steve really at odds over the decision to boot Hulk at the end of Avengers 2 and have whatever heinous thing Hulk does in 2 to justify getting kicked, start the motion of a superhero registration act.
I mean sure, in the comics there were hundreds of superheroes but think about it realistically, even if there were only 8 superheroes in the real world, the government would surely still want some kind of registration.
The only problem with that is I still can't picture Tony agreeing with registration but I suppose you could work up to that or justify it somehow. Anyway, you have a huge rift between the factions, have Stark, Pym, Widow and Shield (W. Maria Hill) fighting the resistance of Cap, Hawkeye, Dr Strange and Fury. Then Hulk shows up, they have to front up to their mistake and be heroes bla bla bla.
I'm just spitballing. I suppose my point is, there are actually a million ways Marvel could make ANY story work for a film, even the worst ones, it just depends how they appropriate it.