I think I'm going to go with the idea of the Mind Stealer needing X successful abductions to win the game. A stolen role is added to their Collection, which will include 3 random roles at the start of the game. Should the Mind Stealer be killed, all roles in their Collection will be randomly distributed to players who have a Vanilla Role of some sort. So, vanilla town, vanilla dopp, maybe Survivor Alien?
Survivor Aliens getting a role would make things interesting. Their win-con would stay the same, but they'd suddenly have some interesting and unexpected role to possibly help them fulfill it.
I think this could work.
They will essentially town-side (to delay the game) until the last night, where they are likely to betray whoever is losing.
Players will know that there is a mind stealer and who it is, which should make for interesting play.
One thought on the Mind Stealer's win-con: What if they needed to steal X 'points' worth of roles, and got tech? That way they could take some investigative tech to try and bag better roles (Wardens, Advanced Dopps, stuff like that), or Defensive Tech to try and say alive long enough to grab enough randomly selected roles? It would certainly make the role a little more interesting to play, although it'd be slightly harder to balance. Still, I could pull the points off of my weights spreadsheet that the perl script uses to generate the set-up, so it wouldn't be too bad.
I think this is too meta.
It would be an interesting exercise for players trying a purely mechanical approach to the game, but it inhibits playing the core game.
Because of the power of the mind stealer, dealing with (including making deals with) the mind stealer is very important. This would push the mind stealer's wincon too heavily into the daygame in my opinion.
A completely alternative approach to mind stealing: Mind Swapping:
This Mind Stealer is an alien with
no win con, tech (I'm thinking 1 large tech), and a full role swap ability
which occurs in the morning.
This Mind Stealer is absolutely lynchable, because it consists of an auto-lose threat to every player every night that it is allowed to exist.
At lylo, this Mind Stealer must side with town.
Players who held this old maid of a Mind Stealer for a night will have gained super-cop information about one or more players, and can bus the new Mind Stealer. Players can prevent the Mind Stealer from swapping with them (e.g. on the last night) by killing it. This Mind Stealer is a reverse survivor, which wishes to extend the game rather than end it.