To tell The Infinity Gauntlet properly they will need to make the film two, if not three, films. Yes, this definitely runs into some contractual concerns. But in this case all parties stand to make a boatload of cash, so I suspect they can figure it out. They really have to go all out on this one. We need The Avengers, The Guardians of the Galaxy, Dr. Strange, & Ant-Man vs. Thanos and his minions. (Personally I'd let The Defenders deal with the local level stuff on their Netflix show).
Thus the IG would feature:
Captain America
Iron Man
Thor
Hulk
Hawkeye
Black Widow
Scarlet Witch
Quicksilver
Vision
Star-Lord
Drax
Gamora
Rocket
Groot
Dr. Strange
Ant-Man
This way the entirety of the Marvel Universe will be represented: earth's heroes, cosmic heroes, mystical heroes, and "Miracle" heroes (mutants).