I'm pleased to report Loki is not going to be in Avengers 2 and is unlikely to be in Avengers 3. Whist I was routing for him in Thor, he became extremely irritating in Avengers. I hope he dies in Thor 2
Why did anyone think he would be?
The only reason he was the villain in Avengers was so that they could claim it was true to the original comics as he was their first villain the team fought despite that...
a) The movie version of Loki is more sympathetic than the comic version which had those elements developed over time.
b) A trickster god doesn't suit the grander way the Avengers come together in the movie hence why it was Thanos orchestrating it.
They won't kill him off, he is too loved. But he most certainly needs to be rested.
I think it'll be fine to keep him in Thor movies as he's important to that mythos and undeniably suits it.