Stepmania and Pinmame are pretty popular,as are Openlierox,and Super Maryo Chronicles and M.A.R.S ( http://mars-game.sourceforge.net/?page_id=8 )
No plans at the moment, no, but if anyone external wanted to port them, we wouldn’t complain 
The biggest complication for new cores is usually timing. Libretro works best with frame-based timing, which is why so many cores are based on emulators. For games that use an internal timer that’s not based on video frames, it can get very complicated.