I think the Cursor*10-inspired single player co-op is an idea with legs. Of course Flashbang recently proved this with the lovely Time Donkey, and next year will see a slick looking side-scrolling shooter, Zeit² from Brightside Games, play with the idea some more. With the ability to rewind time you can help yourself out as you approach trickier parts of the level. There's a trailer below.
Zeit² was a finalist for the Student Showcase award at 2009's IGF, which is always a good sign. The concept: side-scrolling shooter with time travel. It's less clear exactly what the fast-forwarding does at this point - it would seem rarely a good idea to speed time up when fighting a constant assault of enemies from the right, but that shall likely become more clear as more details are revealed. In the meantime, the rewind to support yourself mechanic is one that fits in perfectly to a sideways schmup and could add an intriguing new level of skill. See what you think: