There are some rumors - bizarre, unfounded, rumors - that say that there may be some sort of licensing agreement with Disney on the way. I'll keep y'all in the loop as I find out more.
Seriously though, all that I know about the summer is that there will be more Star Wars sets, a few Harry Potter kits, and two newly licensed Powerpuff Girls sets. It's pretty dark out there, since we're still in December. If I had to guess, we will be able to definitively say whether or not a new theme is coming by the end of January.