Fair enough.
In that case I'm firing away.
1: Has there ever been a "Make a new artstyle" contest? I know it'd be more centered around textures and models, but so many interesting artstyles have already been created just as ideas, what would happen if there was an actual contest around it?
2: Gamemode contest? I've seen so many promising prototypes all over TF2 of alternative gamemodes, like CP Domination and MVM Payload, but I'd imagine within the construct of Hammer there's many more to be discovered! Why not have a contest where entrants are asked to make a map with a "New" (meaning unused by Valve) gamemode? It would encourage people to experiment with making certain game modes viable, and making more finished maps with that in mind.
3: I think everybody would still love another Artpass contest. Granted, this time Valve likely won't interact, but the sheer ammount of ways to detail a single play space is absurd, and it's very fun to try and find your own way of turning this blank template into a real environment...
Frankly, you could even Alpha-ify an existing Valve map, and center a contest around re-populating that space with TF2's artstyle.