Talking about a showcase of Construct's abilities... have you lot played Plants Vs Zombies by PopCap?
It is genius. It takes a very simple tower defense game, its brilliance lies in the polish and minigames. There are many different variations of gameplay within the game itself. Hell, there is even a zen garden mode, where you just water plants and stuff!
Perhaps if the community project would have something like that; take basic gameplay, polish it and introduce different gameplay variations. Since it is a community project, each of us could work on our own variations, which, once polished, would then be integrated.
It just needs a solid base engine and gameplay elements. Not necessarily tower defense - the genre is a bit thick at the moment.