Free for All (A must have for me. The only game mode where you CAN shoot any thing that moves, and everyone)
Team Deathmatch (Faction Based weapons and guys)
Hard Core Free for All (NO, hard core would NOT be copying CoD. Battlefield has it, along with other games)
Hard Core Team Deathmatch
Now for those I would want to have
Maybe some type of predator mode, like Halo's Juggernaut, where everyone is vs one guy, but that one guy is REALLY tough
A Public sandbox. Join multiple people in making the map. Sounds like fun to me, if you want to get more done in a smaller amount of time
Online Co-Op Campaign
A Survive until you DIE! Like Nazi Zombies, or Firefight... love modes like that