Problem: Keeping a very powerful adventurer with a host of friends as a slave makes very little sense. It'd be dangerous to the economy, very risky for Andunor, and they're as likely to kill their owners as work for them.
Suggestion: Level limit the slave collar to level 15, like with Skal. If you're over that level you can only take a prisoner collar (or be reduced to 15). If you're under the level you can't level past 15.
This would make it make a lot more sense why the slave collars are collaring people that can literally carry houses on their back, or call volcanoes, or other epically powerful feats. It also should be easy to do, since it'd be the same sort of system as Skal's writs locking you to 19.