One of my favorite games of all time is the Facebook game Avengers Alliance. I play it daily as well as go on the Wikia to read up in order to see what comes next.
For those who don't know this game the main story consists of you playing as a SHIELD agent with the purpose of recruiting various heroes like Iron Man, Captain America, Spiderman and the rest to take down the bad guys. The story spans 12 chapters so far with the last chapter being the grand finale for Season 1 with a Season 2 coming shortly.
Throughout the first 11 chapters you fight most of the big name baddies of the Marvel Universe like Red Skull, Green Goblin, Crimson Dynamo, Magneto, and a lot of others. The last chapter, however, has you fighting literally every baddie you have fought so far with the exception of Magneto because you actually get to recruit him in the game as a hero who fights the villains because they have resurrected the Red Skull. To see what it looks like just click on this link:
http://avengersalliance.wikia.com/wiki/Chapter_12_-_Titanomachia
This layout is roughly want I see the grand finale being like. Basically Thanos has the Avengers fight every major villain that the heroes have fought in their solo adventures throught the three phases along with their armies like HYDRA army, Chitauri army, Extremis army, and Dark Elves.
If the Avengers survive this enormous challenge then that is when Thanos reveals himself as the one who has been pulling the strings all along. That's when the big finale takes place and something happens that has never happened before in any comic book movie ever: The Avengers lose and Thanos takes over the universe. That would leave room for new heroes to arise from all across the universe to take him down once and for all meaning that their would be many more Marvel movies to come.
I know this is all far-feteched to be sure but Im just hoping that the last battle in the Avengers 3 will be even more glorious than Avengers 1. I think this is the battle that can take place to make that possible.