I love the ideas mentioned so far! I like the idea of giving limitations--limiting us to either shapes, techniques, a specific medium, tools... I would also like to see a KopyKake-prohibited challenge.
A no KK one would be good- but I don't have one, so I don't think of that as much of a limitation .
Perhaps I'm wrong; but I think a Kopykake is cheating in non-commercial applications. O maybe I'm jealous because I don't have one.