As Vivi22 pointed out, the Chitauri didn't just appear out of thin air, and their intentions were no secret. If they seemed out of place to you, I guess you just weren't paying attention, which I kind of assumed from your post but I wanted to give you the benefit of the doubt.

As for wanting them to change Loki into an brawler character, well, that's nice, I guess. I'd like to have seen Ms. Marvel show up and save the day but that didn't happen either. You have one or two good points about Loki being less effective as a known enemy and maybe they should have just used someone else, but they weren't going to change Loki into something he wasn't for the movie. That'd be dumb.