The developers responsible for Rock Band appear to be working on a combat oriented game, if a job ad on Gamasutra is anything to go by.
Harmonix is seeking a Combat Designer responsible for creating "real-time, single-player combat experiences for a brand new IP on next-generation hardware." The ad also points out that the IP will be "unlike anything Harmonix has ever done before".
The ad doesn't reveal much about the game itself, though the Combat Designer will be responsible for elements of the single-player gameplay and AI systems. Which means the game will feature a single-player campaign and enemies will have brains. Excellent.