I think it would be pretty neat if the evil factions (or just some of the morally dubious LLs) could acquire unique ancillaries from defeating a faction, as if they had captured and enslaved or gang-pressed their leaders and notable figures into their service.

Yuan-Bo might give a bonus to province growth or income due to his extensive experience as an administrator, or Alarielle might grant increased replenishment, and so on, and so forth.

If you wanted to be extra you could code it so that if their faction somehow resurfaces via rebellion or post-mortem confederation they are spirited away from the ancillary list.

Also, it'd be kinda neat if some factions could steal ancillaries off of other factions by defeating their armies or using "steal technology" or something- maybe with reduced effect? Though, that'd probably be a pain to implement.