Glad l4d is getting some love.
I enjoy the idea of custom maps, as this game has been out for such a long time that the standard maps are all but obsolete. A vote map function with these may also keep people playing through multiple rounds instead of one and done.
Perks are a good choice but a delicate process. Have to find the right balance between fun and overpowering. Especially if it's part of a versus server
A small group coop server usually has a weird success rate per 12 human players allowed (I.e. The best servers have 12/24/36 slots, exception of 32)
8v8 versus has seemed like a great amount of players to keep it fun but not impossible for either side to win.
Custom skins are very difficult to add, if memory serves me correct. Especially how gmod has a point shop for skins and accessories; downright impossible, but allowing players to use their client side skins could be another benefit and attraction
The Director (AI fucker) has bugs when the slots increase more, so might have to keep an eye on that, which would solve the spawning issue.
Have my input for no reason