Finn Varra: We had a Legendary Riders contest, but that included animals like horses, Battle Cat, etc., not just vehicles specifically, so I would think that a vehicle-only contest would probably be different enough. I'm thinking the vehicle would probably be work enough, so I'd probably not require a customized figure to go with it.
Bob-a-Ferret: I have been considering writing up a base contest ("Bases are Aces" or something) rather than just repeating the larger diorama one (scale-wise, something smaller that would fit inside a typical clamshell/blistercard--think Marvel Select or early Marvel Legends waves.) Because the base would be a smaller thing, I'd probably require a customized figure to go with it. So, instead of making an entire Bat Cave, you'd want to just focus on a smaller section/area of it.
Here's an idea I saw on a sculpting forum that I belong to and thought was kind of interesting: You take three random, generic objects/concepts and then have to incorporate them into your figure in some way. Structurally, in the rules, there'd be three lists, A, B, and C, and you'd pick one thing from each list and then you have to find some way to incorporate them into your figure (kind of like those fill-in-the-blank ad libs, except in 3-dimensions.) Let's say you picked "sword", "triangle", and "undead", two ways you could approach that are to make a zombie armed with a sword and wearing a triangle-shaped amulet around its neck or a rock 'n roll musician with a vampire on their T-shirt, a sword tattoo, and a triangle-shaped body on their guitar. What do you think?
Some other ideas I've had recently are a bootleg/knockoff contest (make screwed-up "counterfeit" versions of existing toys) and a Thundercats-only contest (to celebrate the return of the cartoon and toys.)