For me, this game is about thralls. As a PVE player, collecting thralls is a big part of the allure of the game. With the thrall cap being implemented to limit the number of active thralls per server, include a way for players to swap a thrall to an inactive status to make room for a new or current inactive thrall.
Limiting the number of thralls and adding a finite leveling system will mean at some point you will either need to break bonds with a fully leveled thrall in order to experience leveling again. Do this enough times and you will be breaking bonds with perfect rolled thralls just to experience leveling again. Adding a way to manage all the thralls you place that does not interfere with server operations or PvP aspects would be a good way to keep a focus on thralls.