Imagine a world where casual and hardcore gamers can enjoy games together? So instead of hardcore gamers pretending to like wii sports just so they can spend xmas with their family they actually prefer it as opposed to just going off and playing the best hardcore games such as Skyrim or Fable3.
“Dancing and FPS” could actually work, at least if it's less of an FPS and more of a lightgun shooter. Your avatar would use exaggeratedly cool and groovy dance moves to dodge enemy fire while you strike poses with your gun and shoot foes. Or something.
EDIT: Actually, wait, which one is the casual and which one the hardcore game here? Considering how very challenging real dancing games like the DDR series can be, mastered only by devoted arcade pilgrims, and considering how awfully common (if not omnipresent) FPSes have become, this is a little ambiguous!