^Aside from the bat theme, which would be a harder one to pull off comparatively, I don't think anything with loads of tunnels is a bad idea at all. In fact, I'm really hoping that the next coaster has a few, as SDC doesn't seem to have incorporated any other than the one on ThuNderaTion. Mmmmm, a boulder dash with tunnels...
As far as inverted coasters go, it would be interesting to see if they could build it into a hill or pit like Nemesis. It wouldn't be as environmentally friendly, but they're already filling in gorge's and everything.