There have been a few things about online multiplayer that seem incomplete or poorly executed, aside from the obvious issues everyone has with lag and hosting.
1. Heroes would be so much better of there were a few changes. Everyone that isn't the hero should be either forced to choose one of the default loadouts or a weapons cap should be in place so that you can't use weapons that are on par or stronger than the hero weapons. Actually make it a challenge to get the hero kill. Also, it should be forced spawns, and neither team respawns until both heroes are taken out.
2. Remove friendly fire from TLTK.
3. Add a couple new modes where players are eliminated after so many deaths. Yes, it's boring waiting around for the match to finish, so turn players eliminated into bombers but they don't gain xp for the kills.
4. Condense the unlock system so there is an unlock at each level. Speeds up your trip to the top guns, which would probably keep more players interested in the game for longer periods. And why cap the level at 56? You can keep earning xp, so continue the level progression to give an idea in game how long someone has been in the game. Though, it'd be funny to see the level of hacked accounts with billions of xp.
5. Change the proficiencies in a couple ways. First, the non-pistols need to be changed so that each level achieved takes more kills than the previous levels. It should be greater xp/kill earned at the highest level, not one of the middle levels. Second, the proficiencies should be open-ended, and increase sort of the same way xp leveling does.