I realise that dealing with 'evaporating' followers is a complex matter. Some problems (like vanishing when player leaves the game on the world map) bugs us ever since the followers were introduced. It will also probably take a lot of time before the matter will be dealt with, so...
So I think there should be some way of returnig lost followers introduced for the time untill NPCs disappearance stops to be the problem. I don't know much about scripting (nothing at all to be honest), but I think there could be some kind of 'returning' command included. Maybe a command to 'save' the Follower info at your tent/base, so if he will vanish (but not die), another command could restore him in your tent? Of course there should be some limitation to this, to prevent exploits (teleporting cows to tent instead of bringing them back on foot, multiplying items by saving the fully equipped merc and deliberatly loosing him when he has nothing on them)...
I.E. such 'recall' command probably should have some kind of timeout (can be used once every 48hours?) and would involve loosing merc's stuff and bringing back his HP back to the time he was saved, but it still would be SOME WAY to not loose them completely
It could look like that:
1.Every merc/slave has it's owner id info added, when enslaved/bought/sold
2.You enter the tent/base with an black HtH guy. You talk to him, press say and type ~savenpc. This saves NPC id and info about him. Then later you can - while being in the same tent/base type ~restorenpc - the game checks if any of the saved NPCs still exists and then if any of them has your id. If yes, it deletes him and create a copy based on the info form when he was saved the last time (so basically, you get your merc/slave/animal the way it was while saved for the last time).
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Or include any other way of getting them back. Any.