Fighter thralls are still breaking the PvP experience. They are simply to powerful. I do like the idea about thralls and their leveling, and they are pretty useful when doing dungeons and boss fights.
How about making players immune to other players thralls, damage wise. This way, one could still benefit from the thralls in dungeons and against NPCs, without breaking the PvP experience!
Maybe make it a server side setting, so private server owners can choose.