It's times like these I wish I was still modelling
Incidentally I looked at your mod a while back, when you got that fantastic and amazing new character model working in game. (Whoever made that is undoubtedly an amazing man with talents in all fields, and is likely a killer with the ladies and has sex with them all the time). I remember a post about the approach you wanted to take emulating Vulvas open contribution method, and I liked it.
I think you've got a unique challenge with the gameplay of the maps in this mod. You need to have enough open space to allow for awesome jumps and slides, but enough cover to allow for some duck-shooting and also enough visual interest to make the levels inviting. I think what Sathor said about locations could be really extrapolated. An opulent lobby with staircases (think chow yun fat sliding down that banister) and windows and open areas to slide and jump through would suit the gameplay, and the setting would be in line with the "stylish" idea. I'm thinking James bond, ultra-affluence and such.
Also to allow for a greater pool of contributing artists, might I suggest not setting your hearts on a realistic graphics visual design? There's definitely no need to go tf2-stylized (and I think that would almost be a cliche, think of games like 'the ship' etc) but a tiny bit of stylization - simpler textures and more colour/silhouette recognition based design rather than aiming for grimdark or gritty - would add to memorability and allow a greater range of artists to contribute. I found when I was looking to contribute to mods and I wasn't too experienced at modelling, I tended to gravitate towards stylized models and textures just because you can get away with more.
Also do you have a prop list for modellers? Sometimes modellers just like to model random shit every now and then and having a list of required props and reference images ready helps them overcome the "what should I model?" stage. You might find random modellers willing to drop random props if you have a list with references ready to go.