How about you guys think of a name for a mod(Thats you TBCS not the modders). The modders then have to design a PC around that name, if it doesn't fit the theme then obviosly they would have less chance even if it looked great.
Just to go along with the WC prizes as said before you could have a challenge for the best WC case, or something with watercooling included. I think that could have some sweet outcome. A CPU block,, Radiator, Reservior, Tubing or a Pump would be nice as a prize because they would be able to be used in 'most' PCs, a GOU block wouldn't work so well as there are too many types of card blocks. But a CPU block that fits AMD and Intel boards would be good, like a DangerDen or Swiftech block etc.
Maybe a 'clean' mod something where there is no messy wiring, no ugly colours, something that mixes well together. A few colours that make it look great and not over done.
A prize could be a top of the line CPU cooler and some fans. Then the modders have to build a case with the best air cooling properties, making sure airflow would be well controlled allowing components to have cooler air instead of warm air lingering inside the case.
A game maybe could be the prize, the modders then have to create a design around that game. So a racing game could have a case modded to resemble a car, or have something that makes it link in with the game. etc.
I can probably think of many more but i can't be bothered at this current time