Here’s what I’ll call the standard way to handle M-U spells: you have a spellbook, and the spells in the spellbook are what you can choose from to memorize. If you want a new spell, you need to find a magic scroll or a whole spellbook or invent a new spell through rigorous research. But the spell is a magical thing, it detects as magical, and you can’t just hire some scrub scribe to transcribe it.
This makes spells less common because there’s an investment in copying them.
And I would say in general that fun is had when the PCs gain new spells slowly, and when enemy M-Us have spells they don’t have yet, and when the enemy M-Us can play some of the same tricks the PCs use.
It’s also kinda fun to be able to sell your spells if you’re in a bind. But if a PC can make this “free money” by selling his spells, why wouldn’t he? If PCs are so free with their spellbooks, why are NPC M-Us so secretive?
I think it’s espionage; if you have a secret, you are more powerful for it, in direct relation to how few other people know the secret. And in terms of magic, the secret itself has physical power separate from social power or the ability to take advantage of opportunities because of extra knowledge.
So, you can also trade spells with other M-Us, but beware: every M-U you give a spell to is a 1 in 6 chance that every other M-U will have access to it. If you sell it 6 times everyone is gonna be running around blasting that stuff. And that’s a bad thing if the spell you sell is one you’d rather not have used against you.
This works better if there is a standard spellbook everyone starts with. I’d recommend maybe 3 good spells and 3 lame spells per spell level up to 6th. Beyond that anything is uncommon. So that every M-U isn’t walking around with a 10k gp spellbook, it would also help if spells had virtually no cost to transmit. What if a spell was more of a set of instructions, a way to exploit your knowledge as an M-U and your spellbooks? Then selling it isn’t a matter of recouping material costs, it’s a matter of how badly the other guy wants it.
So, we now have a situation where your M-U PC looks around and sees a bunch of other M-Us around town with their own agendas and their own mysterious powers. One may be a master of the Webs, and another may exploit the mysteries of the dreaded Ray of Enfeeblement. And of course nobody tells what all their tricks are, and rumors abound.
An M-U who emerges from the dungeon with new spells will be approached by emissaries fro many wizards wanting to trade or buy, but most offers will be insincere: traps, or lowball offers, or just attempts to figure out exactly what the spell is. Some may be just and true, and might remain good friends for some time. Of course, these good friends are also somewhat likely to pass the spell on to others.
The only people who will not add to the dissemination chance are PCs. Trusted henchmen add to it. Your mother adds to it. NPCs gotta pay the bills sometimes.
And if a spell gets disseminated, it gets added to the standard spellbook for PCs and NPCs alike, with an X in 6 chance next to it until the chance is 6 in 6 whereupon it just is always present. Any potential buyer for a spell (after the first one) will have a chance that it’s already in his book. A spell simply can’t be sold more than 6 times because there are no more buyers, and the more times you sell it the more work it is to find a buyer (and the more likely the buyer is undesirable, unfriendly, more likely to assault or cheat the seller).
Note that in my experience, players typically share spells freely anyway, so this gives an interesting choice between keeping a spell secret or benefiting from selling it.
I imagine this working on a small-scale campaign because M-Us travel around in a small area and make a few desperate trades if necessary. In a globetrotter campaign, I assume NPC M-Us are just as mobile, so information disseminates just as quickly on a global scale as it does on a local scale for a local campaign. Yeah, this means if you sell Fireball in Cormyr there’s an instant 1 in 6 chance every Waterdhavian M-U knows it. Magic is magical!
And, of course, my personal “guaranteed spell list” would not include the most useful spells like Fireball and Fly, but would include less-useful ones like Levitate and Magic Missile. Just enough to get by without adventuring – except any M-U worth his salt will DEMAND to adventure to get new spells!
And if the PCs capture an NPC spellbook with Fireball, and share it among themselves, and agree not to share it – imagine their chagrin and suspicion when one of them ends up taking one of the lavish offers from some NPC and M-Us start showing up with Fireball! Who did it?!
Spell creation is a process that takes years of labor for a high-level and high-INT M-U, and may end up being fruitless. Just like mathematicians working on difficult math problems that lack proofs and solutions.
The repercussions include de-magicking the world. When only a few rare M-Us know Continual Light, you won’t expect to see Continual Light along every village street. If you want a spell to have that kind of widespread Ebberon-style impact on the world, include it in the auto-spell list.
PC M-Us should be a little happy about this since they actually start with a lot more spells (albeit not great ones) and they can feel like they have something rare and powerful even at low level if they find a new one. Plus the interesting choice of disseminating it or not.
Finally, this can replace Chance to Learn Spell if you don’t want to keep it. But I like the idea that an M-U can come across a spell and just not grok it. No matter how hard he tries. Makes every M-U different without needing specialist classes.