Avengers 3.
That's the rumored Infinity Gauntlet movie. Hence why they put each stone into every single movie.
By the time Avengers 3 rolls out, all of them will be shown and Thanos will suddenly have all of them.
We literally just went over how they've only appeared in four of the movies.
Point. Sorry, I was thinking wrong. But they are putting the gems in a lot of their movies, especially since Thor 1.
Gauntlet in Thor 1
Gem of Space in both Captain America 1 and Avengers 1
Gem of Reality in Thor 2
Gem of Power in Guardians of the Galaxy
Gem of Mind is rumored to be in Loki's staff in Avengers 1
So that leaves 2 more gems.