This would be something like the grid in insurgency or squad, where you get a square of say 9 maps and 20 seconds to pick one, then the one with most votes gets played.
Gate behind a sv_mapvote flag or something like that
No visual design exists yet. Should probably borrow visual elements from gun selection menu.
Also need to figure how to bake in previews into .BSPs, but that's probably not too hard. Or just have an assets folder with preview images. Yeah, that's probably better tbh.