Ideally (to me), potential companions that are not invited to join the party to begin with or that are dismissed at some point in favor of other companions, continue to find a path to salvation on their own, possibly aided by other non-invited or dismissed party members that align with their personality, crossing paths with the player every now and then.
Players changing their mind about those non-invited/dismissed companions, who wish to later reintroduce those companions, would need to overcome the disapproval rating of those companions (Oh, you thought you could do without me, huh? Treating me like a pet that can be discarded at a whim.) by completing one or more tasks for that companion.
I'd be especially excited if there's a disapproval threshold that could be reached that causes the companion to not only become truly locked out from being a potential ally, but to actively become an enemy that opposes the player.
This may my recent Crusader Kings 3 playing time talking, where your vassals have an approval/disapproval rating that you need to manage, and where those vassals can start rebellions against you if the disapproval is too high for too long.
