I don't think summer 2015 will be as crowded as it seems. I'm fairly sure Fantastic Four and Terminator will still be in limbo by then. There's going to be 2 big, billion dollar films that summer: Avengers, Batman/Superman. Mockingjay will probably be earlier in the year and Bond 24 and Star Wars closing the year out. I can't see Antman being treated as a "summer blockbuster" kind of movie and Jurassic World should be fine in its June slot.
This year had a VERY crowded summer and I dont think we're likely to see this kind of claustrophia again. We had Ironman 3, Star Trek, Man of Steel, World War Z, White House Down, Pacific Rim, The Wolverine, Fast 6, RIPD, Elysium, and After Earth. They should have spread these films out throughout the year so they could breath, they all, with the exception of Ironman, could have probably made more money if they had spots where they could breath.
From the looks of 2015 will be HUGE but it will be spread out as opposed to this year.