I was kicking around a setting pitch recently, something like
The fire from the sky has ceased, and the earth has mostly stopped shaking. In the isolated hamlets and manors that escaped the destruction, peasants and nobles alike dare to hope that perhaps the Wizard War is at its end. The ambitious wonder what treasures and lore lie ripe for the taking in ruined towers, while the wise worry what warbeasts and fell engines have yet to be released.
Such a setup seems like it would have some nice properties - it fits the post-apocalyptic assumptions of old-school D&D by making the apocalypse explicit. It gives me a lot of liberty to place fantastical ruined environments and landscapes close to utterly mundane surviving settlements, rather than having to go hard on realism/consistency in my dungeons like I do in most ACKS campaigns or having to go interplanar like in the Rathell campaign. It's a good excuse to not have any class I or II markets, which in turn prevents the accumulation of large numbers of spies to break hijinks, makes it hard to sell magic items, etc. And it supports a rather Iron Heroes relationship between player characters / civilization and magic; magic can be rare and scary.
It got me thinking that maybe it would be interesting to treat magic user as a side-class. It's not something you can start with; all the master wizards were involved in the Wizard War and are dead or worse. You gotta go dig up a book and read it, and that lets you become a 1st-level wizard as a side class (and that book becomes your spellbook). Then to advance further as a wizard, you reverse-engineer magic items, destroying them to gain XP. It would be appropriate to have reading tomes grant XP, but then if you have multiple wizards in the party they could pass them around and that gets ugly - destroying items has a finality to it. Plus you can get still new spells reading tomes, so it's not like books aren't useful. And if you make reverse-engineering items take time (like a couple weeks in-game), then your wizards will have more down-time than your non-wizards and will end up with less adventuring XP in their main class, even though they're not actually splitting that earned XP between their classes.
And then because nobody likes wizards (not even other wizards!) you get a reaction roll penalty scaling up with level.
Practical complications here: no sleep to win hard fights for 1st level parties. Need XP values for items. If thief is a side-class and wizard is a side-class... are you only left with fighter and cleric as your "base" classes? Or do you make cleric also a side class (maybe that also gets XP for destroying "profane" magic items). Do I want to deal with fighter/wizard/thieves or just limit to one side-class? How does casting in armor work if every wizard is also a class that gets armor? If you have a very limited set of base classes, what do you do with with stat-lines that have eg bad Str and bad Wis?