Broadly speaking: nontraditional hullform challenge.
I've personally been mulling over a modern SWATH FFG, but I'd be equally interested to see regular catamarans, trimarans, surface effect ships, and whatever else.
It would be hard to do this justice without at least a top view and possibly a bow or stern view. Not sure how folks would feel about encouraging or even requiring one.
VERY VERY IMPORTANT EDIT: not before, like, February please
I just bought a house and I have a lot to do.
Sorta jumping off this one, Cold War Experimental ship challenge. Design a ship for the purpose of demonstrating new naval technology being developed during the cold war era.