First, i agree with what torkell says: Some players (like me sometimes) play with a friend/little sister that gets pwnd everytime at 2 computers instead of split screen. So maybe if it would be possible you could do one player for each computer ip or something (I dont know much about all those different ips so im might saying something stupid )

Second, the idea of map voting is great. You could turn this on and off with something like /mapvoting on/off. However there would be one problem: How could you know wich maps there are on the serverlist? Or can you choose one map from the hosts file?

Third (im so good at this counting), when you raise the resolution wont you ruin a lot of levels out there?

Last, a suggestion for me: What about a score screen at the end of the level instead of the black screen? So you could check the kills/dies/damage to other players/etc when you finished the game. This could be fore like 5-10 seconds and you could be able to turn it on and off (and ofcourse when you press enter you will skip this, else you'll get more problems with it). And for the map voting you could also put a screen with all maps in a list and a screenshot of the whole lvl + name + filename + tileset + music (ok, the screenshot might be a little unwanted). So the voting screen would come after the scoring screen and you would have a limited time too vote.