Double flings. Not twitchy ones.
There were plenty in Portal 1 that weren't twitchy and not many in Portal 2 at all.
I like flying through the air between portals.

If you've played the Portal: Prelude pack, basically don't do any of the stupid twitch stuff in there (respect to the guys who made it, they put in a lot of work, but the maps/puzzles just didn't feel Portal-y).
I think there's a lot that could be done with those suction things, they didn't get used much. Maybe redirecting gel around, if they'll do that?
Gel on boxes is fun. Making them bounce to release buttons/break glass and slide down slopes/etc.
I mentioned it before, but in the single-player there's no gel/light bridge puzzles (I know there's some in co-op though). Timing with light bridge activation + gel splatter could be interesting.
Not guaranteeing any of these will play well, but should be worth checking out.
