I would keep the current multi system, but make it more nuanced.
Winning the round is what counts, so that will increase multi by 1x.
Killing a player would increase it by .2x
assist or horse kill is .1x
So you get 2 kills, 1 assist, and win the round. So the next round you have 2.5x.
OR, maybe just xp/gold bonuses for killing/assisting with an extra bonus for winning, or something like that.
Whatever the system, I hope it rewards individual skill as well as winning the round.