Looked into it a bit more, and the Thanos rumors are pretty heavy but have a lot of holes in them. Most of the rumors say that Loki is collecting and offering Infinity Gems to Thanos as long as he attacks Earth, and Thanos gave Loki the Skrulls (which go by a different name in the movie) to help in the battle. However, other rumors say that the Skrulls are not working for Thanos, but simply are following Loki because he informed them that the Cosmic Cube was located on Earth, which they want to possess. Really, the rumors are all over the place it's hard to narrow down which ones may be valid and which are just fanwank.
However, talks with the director and others in the movie have confirmed that any other villain won't be the main antagonist, that will still remain Loki. Whoever the other villain is will probably be a blip at the end if anything.