"In ages gone," the Sage had said, his eyes fixed on a low star, "a thousand spells were known to sorcery and the wizards effected their wills. Today...a hundred spells remain to man's knowledge, and these have come to us through the ancient books..."
The Dying Earth, Jack Vance
We all talk about the Vancian magic system in Ye Auld Game, but 99% of the time we are talking specifically about "fire and forget". Vance's strikingly original presentation of memorizing and forgetting spells is certainly one of the most unique in literature, but there is actually more to Vance's magic than just that. The quote above is the one that always sparked my imagination. These arrogant wizards know that, in fact, they have a tenth of the knowledge of their predecessors and, rather than convince them to pool their resources, it actually makes them guard the few spells they have both zealously and jealously.
Now, that's something I could do with more of in YEG. So, imagine this:
Whatever iteration of the game you are playing, the 1st level Magic-User spells are the entire syllabus of known spells.
Fly? You wish. Fireball? Who can shoot exploding balls of fire; this is magic sonny, not comic-books.
Of course, everyone knows that there are, in fact, other spells out there. Every first-year sorcerer has read that bit of Laconius where he mentions the Miraculous Mantle of Obfuscation, which renders the user invisible, and the intemperate uses to which he put it in relation to the Witches' Coven of Outreterre Or known the frustration of studying that antique blabber-mouth Quinquarine, who, over the course of several volumes, promises to reveal the formula obscurely referred to as The Perambulatory Revelator of One-and-All, only to have no copies of the final volume survive the bonfires of the Irenian Orthodoxists. Or stared at the great tapestry at Biancule, depicting the turning of the invasion of the Mauvrian Hordes, and wondered what incantation allowed the fabled archimagus Villondro to enmesh the Mauvrians in gigantic spider-webs?
So, of course there are more than just these twelve spells. That's why you are crawling down a hole in the ground with a party of cerebrally-challenged bravos, cut-purses, and roustabouts, facing death a hundred times over in the form of goblins, traps, and pneumonia, instead of staying in a nice cosy manse somewhere, casting Charm Person over and over for a hundred crowns a pop. Because there are more spells out there.
And whoever finds even one of them, is going to be star among the thaumaturgical-set. Seriously, people are going to be hitting you up right, left, and center for just a peek at your grimoire; the grimoire that contains the only known copy of Ariste's Vertical Realignment in the world (even if some slack-witted copyist wrote it down as Levitation. But, hey, that error kept the thing lost all these centuries 'til you found it, so be nice to the guy). You now become famous as "So-and-So the Levitator" and anybody who needs something vertically-realigned has to come to you; either to pay for the privilege or to try and steal your spell (recall, in this context, the bit in The Dying Earth where Turjan crashes the chambers of Prince Kandive).
The Referee is going to place far fewer scrolls containing spells above Level 1 in this game because even a single Level 2 spell is a major treasure. Spell-Scrolls retain their value , of course, because of fire-and-forget. The Referee is also much freer to cut off the spell list wherever he likes. Don't like the Wish spell? Fine; it never need come up. In fact, if you don't like the whole concept of spells above level 6, say, then that's easy enough too.
One additional option is that one may want is to grant all MU's the entire Level 1 list in the books at start, in part for compensation and also because it sounds right to me. I'm influenced here by Stephan Michael Secchi's Talislanta. There were some thirteen "Basic Spells", including the Spell of Eldritch Power, the Spell of Conjuration, and the Spell of Radiance. Anyone who had made it to 1st level had mastered those. As Secchi puts it:
The following list of spells forms the basis of Talislantan magical tradition, and represents a body of knowledge common to all practitioners of magic, regardless of race, nationality or profession. Apprentice spell casters spend years learning to master the complex verbal and somatic components of these powerful incantations, the origins of which date back to the Age of Mystery. So ancient are these spells that their authors' names have long since been forgotten.
Talislanta Handbook and Campaign Guide (1989)
At that point, one could have an entire career in magic with just those thirteen spells. Anything beyond involved digging lost librams or stealing from someone else. And then there were the big prizes: the spells of the Archaeans, the originators of magic, who were able to craft spells that far exceeded later ones in terms of horse-power and scope.