This might be something better done with Mods perhaps, but I really like the idea of this and wanted to posit it in case it had some interest from Radiant.
I had this idea last night of some scanarios potentially leading into special one-off professions. Imagine a scenario not dissimilar to the Goblin King one - skeletons start turning up at your town occasionally and attacking. You fight them off, eventually muster up some troops and head out into the wilds, and finally find a Necromancer in an encampment who is making skeletons and sending them after you. You battle him and kill him.
When he does, he drops his spell book. This is a talisman just like a carpenters saw or hunters knife. You can use it to promote one of your hearthlings to become a necromancer. This consumes the book, so you can only do it once, and it’s random if it appears in your game at all, so it’s a fairly rare and cool thing.
Your necromancer can now create skeletons (perhaps using goblin trophies, assuming they’re analogous to the goblin’s corpse. Alternately maybe bodies don;t vanish immediately when they die anymore?). Skeletons would be a little bit like hearthlings or the KS-promised golems. They act as workers and do not need to eat, but they move very slowly. Give them weapons and they can fight etc. But make really really sure your necromancer never gets killed, because if he dies, you lose control of all the undead he has created and they start attacking your town etc, so there’s some danger to having them.
That’s just once example (but I really really like it). It would be easy enough to come up with a few other scenarios which could randomly occur and lead to a custom class being available in that game. Another example could be a goblin wolf-riding horde scenario that leads to you being able to train wolves for your footmen to ride (but lose the trainer and the wolves turn feral again and you have to kill them),
TL/DR: The idea is that there is a small set of special classes that aren’t commonly available, but which may or may not become available in any given game you play. This would increase the excitement since you never know what other possibilities will become available, and will encourage replay value because you may have to play a ton of different towns/games before you see everything, whereas the built in classes are available in every game every time so you’ll eventually have “been there, done that” sooner.