For Poltergeist, anything beyond cantrips would fit the bill, really. Even being at-will, I'd feel like I'm wasting one of my invocations.
Maybe split Poltergeist into two invocations of different grades, one lower, one higher. The lower one could be Greater Mage Hand and...Unseen Servant, I suppose. The other one could be full on Programmed Image with some changes to the duration. Maybe you could create more than one Programmed Image with each casting, like 1/CL, or 1 per 1/2 CL, but they only last 1 round/level.
Don't have any solid ideas at the moment, but something like that might be interesting.