Had an idea for awhile ... after gaining a level, instead of players choosing new spells freely for their spellbooks, they're limited to only the spells they either find while adventuring, what they can buy in town, what they can learn from NPC connections, or what they research.
They get a choice starting out, of course, in the 0 and 1st level spells. On the other hand, perhaps the GM has a somewhat limited list of known 0 and 1st level spells that are allowed.
I get a kick out of this idea, because the PCs get to recover lost knowledge in addition to gathering gold & jewels in their adventures. Said another way, getting access to previously unknown magical lore is another reason to adventure. Plus, as GM, you could have different geographical regions with mages & clerics in various lands specializing in certain schools or faith traditions. To learn "new" spells, the party has to travel to other countries and seek out this knowledge.
Maybe the spells also get culturally appropriate names. 'Fireball' in Osirion is instead called 'Pharaonic Fury', for example.
Just an idea ... whaddya think ??? I can hear players groaning about losing some choice, but I find a lot to recommend this idea.