Imagine how fun playing Halo multiplayer was??? My personal favourite was Halo 3, I just LOVED the variety in maps and weapons.
I would love to be able to do a 2D sidescrolling version of this. I think a big part that is missing from a lot of the existing battle games (Smash, Battleroyals), is the ability to navigate maps through portals. I love the idea of having multiple rooms/access points. Being a 2D game would be slightly easier to build than 3D.
It could probably just start off as a small game with x people in a room (maybe up to 4v4?), and then we can grow accordingly.
I think MVP should obviously have core multiplayer functionality, and also have a focus on multiple weapons, including melee and range, as well as movement. I don't see characters inherently having skills, but they could.