Verbs.

Sometimes, I like coming up with new ways of coming up with ideas. Check it out:

Games are ultimately built on verbs, right? In shooters, you shoot. In platformers, you jump. In Braid, you turn back time. I think interesting games often have interesting verbs.

So... maybe to come up with ideas, all you need to do is combine unlikely verbs. Next time you need a game idea, come up with 50 different verbs, then start mixing and matching and see what comes out.