Would it be hard to implement spectator mode and leveling/achievements into the game like modern games have?
Spectator mode basicly is that you see what's happening ingame through another player's eyes, you can switch view to another player at any moment in time or choose to join the game. Not the way other ports are doing it, they make it hard to actually follow the game.
This is a useful feature when you plan on streaming live tournaments you're hosting.
Leveling up/achievements could be something giving people the incentive to play Doomsday over any other Doom port out there.
How fun is it when you see your name ranked on some highscores system? You could even make this use an API so people can create their own highscores/achievements for their own sites.
I hope you like these ideas as much as I do