The elo system still needs work. It does a fair job of estimating player ability, but as you said people can farm elo depending on game mode and opponents.
-Needs diminishing returns for repeated kills on same players (to prevent farming) that doesn't reset when a player simply rejoins a room. Maybe it resets every 15 minutes, so TDMs aren't affected as much.
-Re-balance elo gains/losses for each game mode. Elo gains in duel are too high compared to other game modes. Maybe diminishing returns will help even this out. TDM players can't compete. TDM and duel should be on equal grounds IMO
I think this would be a good place to start... Will eventually need to look into reducing elo gains for large elo disparities (1500-2000+ point differences) and creating an elo per account (that just uses the highest elo) so people can't simply reset/abuse the elo system with new chars
