I think a multiplayer mode that could capture the scarcity and isolation of the Metro series would be perfect. Don't make the maps too big so the modes could be small like 2v2 up to 6v6 at most. Make the maps dark and in the tunnels for a few and above ground for maybe a couple of others. I think too if the above ground maps had a system where you had to worry about your gas mask and filters would be interesting. Just a thought
A very good map for the multiplayer could be the one of the Frontline level. there could be fights going on under the bridge and a huge meatgrinder on the bridge!