WARNING: AGE OF ULTRON SPOILERS AHEAD!!!!
I have watched Avengers: Age of Ultron and there is one scene that has helped me get closer and closer to proving my theory for Phase 3. When Marvel held their press conference to announce the Phase 3 line-up I spent a few days analyzing it all the way through and found something very interesting.
If you look at the dates closely you should see a pattern. Notice it yet? If you don't let me tell you. In 2016 there is a May and November date. In 2017 there is a May, July, and November. 2018 has a May, July, and November. In 2016 there is one month missing that could be hiding something... a secret project that Marvel does not want to reveal. I have been looking into this a lot and now that Age of Ultron is out there is reason to believe Marvel really does have a secret film in the works. One involving an enormous, green, rage monster.
There is a scene in Age of Ultron that shows the Hulk jumping into a Quinjet and riding it high above the earth to keep Natasha and everyone else safe after his recent "code green" as Banner calls it. Now this scene does not show Hulk going into space but I think you can guess where this is going. Planet Hulk may be coming. There are little pieces of evidence that show this could be a possibility. If Marvel is doing a Hulk stand-alone film this would be the way to go. After only having one film in the MCU for hulk I think it's time Marvel makes another with Ruffalo as the lead.
Now if you have seen Age of Ultron you're probably saying, "Fury told Natasha that there was a Quinjet sized object in the ocean in stealth mode and that Hulk swam to Fiji." True. Fury did say this but he did not confirm that it was the Quinjet and maybe he is trying to comfort Natasha and keep her from the hard truth. Letting Hulk leave also would work for his surprise return in Infinity War.
If Marvel is hiding something this movie could be a huge announcement and get fans excited for yet another Hulk film. In my opinion Planet Hulk is one the best Hulk comics I have ever read and it's animated film was awesome. Putting this on the big screen would crush records and give Hulk the movie he deserves.