I think it would be a mistake not to have him in Avengers. Beyond the fact he was teased in "Avengers" and most moviegoers are expecting it, there's the Thanos-assistant line:
"To [attack the humans] is to court death."
If you follow point a to b...
1: He is going to attack the humans (I.E. Earth)
2: Guardians of the Galaxy will most likely have very little to do with earth.
3: Thanos will likely be in the Avengers to attack earth.
Now, this is not airtight, I know. GOTG may deal with earth in a tangential way, particularly since Star Lord is human. Still, I think it's a good indicator, and like I said up top, people will be really disappointed if Thanos isn't in Avengers 2--even though I was really hoping for The Masters of Evil.
This is what I think should happen. Remember how they used Thor to set up Loki and then they had him all ready to be the badaxe villain in The Avengers? Well, do the same thing with Thanos. Set him up with Guardians of the Galaxy and then have hi invade earth in A2. It's so obvious as an additional way to create anticipation for the sequel.
I said it soo many times before. Thanos schemes and level of destruction is so vast. Having him be set up JUST to launch an attack on Earth, to me, as a HUGE Thanos fan, would be a slap in the face, and I would be very, very, very disappointed. However, I think that is not the case. Based on what was said at the beginning of the avengers, Thanos' objectives are on a universal scale, and it will most likely be the avengers trying to stop him from whatever his plan will be.
I highly doubt that Thanos will just...attack the Earth, or if he does, it will not be his climax.
he is bigger than that.
Btw, most of us aren't saying don't use him till 3. We are all saying keep his deal getting bigger and bigger, and his presence getting more and more known, as he stays behind the scenes somewhat, leading up to a climax in Avengers 3. As Sam seems to think, it will be a Thanos trilogy.
I think its either going to go
Avengers 2: Thanos
Avengers 3: Thanos
Avengers 4: Ultron
Or
Avengers 2: Ultron with Thanos still in the backround, but becoming a character noticeably, but still minor, and played off, (maybe plays a part of reviving Ultron or something) the audience would know something is being set up, and at this point, his presence is known in the MCU
Avengers 3: That big pay off with the climax movie of Thanos' schemes,
I am leaning more towards something like this, in terms of preference,
but who knows, maybe we will get Thanos in JUST 2
but I expect the Infinity Gauntlet will come into play.
With it being teased, and don't forget, in the post credit scene, why bother to show his hand? lol, why else would they show his hand, besides the fact that he does not have the gauntlet?