First I think you should be able to buy collars from Grouse at the price of 100 caps, and are simply paid 200 caps when a slave arrives (Or are given the collar back). This way you could buy multiple collars and enslave multiple people at once. If a slave dies you should also be able to retrieve the collar.
Secondly I think that slaves should become followers so that you can escort them, this way you can protect them, for extra assurance they should be made into a faction that is neutral with everyone if the player is nearby (the player is a real threat). There could also be a tracking quest too so if one dies you can go grab the collar or if one goes missing you can track them down.
Lastly there should be alternatives to the (VERY) irritating to use mesmetron, I'm thinking flashbangs, tear gas, tranq darts and maybe even a special baton.
I might consider taking this up myself as a break from Blood on the Sand so if anyone takes this up please tell me.