The Vanilla mutator would be a mutator that would undo most of the changes Loka makes to vanilla PvP(hence the name "Vanilla"). This would mean no shield cooldown, no Loka damage buff, allowing you to eat gapples, no crystal cooldown, etc. The only changes I would see most likely staying when...

Mutator that just puts you on a horse. Would go well with the new spear coming to Minecraft (they announced it in the live, for anyone wondering what it does it just deals more damage the faster you move.)