Let's just use common sense here.
1. Fury end credit in IM 1, then appears in IM2
2. Thor's Hammer end credit in IM2, we then get his movie.
3. The Cube end credit in Thor, then we see it in Cap.
There is no way they're waiting until A3 to give Thanos something to do. I fully expect him to become more involved in the situation somewhere between T:TDW, GOTG, and A2.
Again, I think he will be in both. I don't understand why a lot of people are of the mindset of him "showing up as THE villain" thing. It seems pretty clear to me, with all the speculation as to where he will appear, that he won't just be a "villain" but a character. He will have a role in gotg, and he more than likely will have a role in A2. Even if they are big roles, that doesn't mean he IS the villain.
Just because he MAY not be THE villain in A2, doesn't mean he couldn't have a huge role in the movie.
The people that think he will be in A3, myself inculded don't think that will be when he shows up.
Someone please explain to me, WHY can't thanos have a big role in these movies, but not be the THREAT of said film, but not have his climax till avengers 3? that's how I see it. He'll be there in gotg as a main character, he'll be there in A2, there may still be stuff going on, on earth, but he'd still have a decent role..maybe he'll appear in other movies in phase 3, and then he has his conclusion where he is the ONLY threat in avengers 3...at that point, he wouldn't be teased up until then, cause he would have been in plenty of movies.
i think thanos is going to be the first villain in the MCU who isn't just a villain, but a reoccurring character