Once we approach Ant Man in 2015 after Avengers 2, we make our way into phase 3, from there we are expecting to see about 4 to 8 more heroes joining the Avengers against Thanos and the Chitauri.
Thanos is indeed as powerful as people say and as leader of the Chitauri and is being saved for the grand finale, he is indeed the main villain of the entire Marvel Cinematic Universe. After he gets taken down along with his henchmen called the Other and his entire army the Chitauri, the Avengers are definately going to have a big celebration for their ultimate victory
And very much so without a doubt with Avengers 3 set to be the conclusion of the Marvel Cinematic Universe, Stan Lee will or maybe sure to appear before or after the end credits saying "Nuff Said".
Even though the script or a trailer for the film has not been shown yet, I can still see the movie being so very epic that it will be unlike any other film anyone has ever seen. For now we have along way to go.