There have been other innovations, but they're currently very experimental or there were significant design flaws that effectively made the technology non-viable.

Gyrojet ammunition was found not to have any significant benefits for its drawbacks, and caseless ammunition was pretty much the same.

I know some weapon systems are messing about with electrically triggered munitions (the Metal Storm system comes to mind), but in general I agree that there's not much more for chemically driven firearms to develop, which is a different issue to them being the currently most effective personal weapon available.