I’ve been playing with an idea for several campaign settings I’ve been thinking of. I keep thinking I read it on the forums here, but a search didn’t turn it up, so:
Locked campaign classes.
For several of my campaigns, I think it would add a dimension of exploration and mystery to “lock” certain campaign classes until they are “unlocked” in play, either by finding and recruiting as a henchman a NPC of that class, or simply by discovering the appropriate location or faction, and/or opening up good relations with them.
For instance, in my Crimson Sun campaign, there can be no PCs created or henchmen recruited with the Half-Giant Brute class until there’s a “source” of Half-Giant Brutes: they’re all the magically-created slaves of Sorcerer-Kings, so the most obvious method would be to kill a Sorcerer-King and topple his regime. (In this case, e.g. being given a half-giant slave by the King or his Templars, who serves as a henchman, wouldn’t really “unlock” the class). I might similarly ban halflings - or halflings other than Halfling Renegades, at least - until the PCs have crossed the Ringing Mountains and established friendly contacts with halflings in the Forest Ridge.
In another setting, for instance, Paladins might not be available until the PCs help re-establish the Order of Paladins in their ruined keep; or elves or dwarves might not be available until the PCs have re-discovered their sole remaining/known sanctuary in the setting; or bugbears aren’t available until the PCs have made an alliance with the bugbears.
In other campaigns, it might be simpler: e.g. Witches (the mystic guardians of the Southland, or whatever) become available once the PCs meet and recruit a Witch NPC placed by the Judge.