In my previous post, I introduced a concept I call "spell mastery", which allows magic-users to spontaneously cast certain spells. The rule, lifted from 3rd edition D&D and modified, seems workable enough, though it hasn't yet been playtested. I'm still trying to figure out exactly what process characters will have to undertake in order to gain spell masteries, but I do have some ideas.
I thought about awarding a set number of masteries based on the character's experience level, but that feels a bit dull, not to mention that it would mean that literally all magic-users would have mastered at least a few spells. I want spell mastery to feel more impressive than that. (Still, I'll probably give starting characters a mastered spell or two, most likely based on their Intelligence score.)
I also dislike the idea of creating schools or domains of related spells from which a character would have to choose. I thought long and hard about it, and in the end I feel that imposing a classification system on D&D's already reliable and mechanistic way of handling magic is just making the whole thing feel even more like a science (or worse, a technology). I've got no problem with players choosing to master spells that are related thematically, but I'm not going to force them to - if somebody wants their character to master ventriloquism, anti-plant shell, and teleport, I'm not going to object.
So, I'm thinking about making characters seek out a mentor in order to master spells. These mentors wouldn't just be old wizards that charge for magic lessons, though. I'm picturing something more like Fafhrd and the Grey Mouser's mentors, Sheelba of the Eyeless Face and Ningauble of the Seven Eyes: mysterious and inhuman figures whose goals are often inscrutable. Or they could be like the "Spirits, Partly Evil and Partly Good" described in the old demonology tract, The Goetia, who impart specific knowlege and abilities to those who call upon them, but are inherently unruly and untrustworthy (and will often seek to harm an unprepared or arrogant student).
Seeking a mentor would not be a quick or easy task. I could handle such a thing as a full-blown adventure - "Quest for the Cave of the Hidden Master" or what have you - but then I risk making the magic-user and his acquisition of spell masteries the subject of the entire campaign, relegating the poor fighters, thieves, and the rest to the role of bodyguards and henchmen. (There's already an RPG for that: Ars Magica.) My current thinking is that the best way to handle it might be something like the carousing table, to be used in the downtime between adventures, in order to keep one character's spell-questing from distracting from the meat of the campaign. Some kind of formula that lets characters convert earned gold and invested time into a chance at spell mastery, with a nice random mishap chart, I think...