It's been a couple of years since the last retro-clone was released (well, unless you count Pathfinder). We've seen clones for whitebox D&D, B/E D&D and even the BRP system.
What do you think will come next? 4E's limited GSL and great popularity makes it a candidate. There's no 2E AD&D yet, and surely that wouldn't be too hard to reverse-engineer. Or is there some other game system worthy of the retro-clone treatment?