1. Health.
maybe a small health boost of a few hearts for killing someone, then it's less easy for people to pick you off when you're on low health from winning a battle As well as giving you a bit more of a chance of killing that person. (Only Applies to free for all, Would be a bit OP in anything else).

2.Team scrambling
Maybe a command that evens out the teams during a game so you don't get 4 v 1's.