"The tomes which held Turjan’s sorcery lay on the long table of black steel or were thrust helter-skelter into shelves. These were volumes compiled by many wizards of the past, untidy folios collected by the Sage, leather-bound librams setting forth the syllables of a hundred powerful spells, so cogent that Turjan’s brain could know but four at a time."
-”Turjan of Miir”, Jack Vance
There is no specific mention of levels and slots. Spell levels is clearly one way to count "how much" magic Turjan's brain can handle and "how much" magic each spell represents, but it's not the only one. A spell mechanism could very well be created around spell points which can only be used during the morning hours when the wizard's mind is fresh and recovered through sleep. Spells would then lack a level per se. A character can have 10 points which are spent on five spells each requiring two points or on two such spells and two three point spells, etc. Maybe some points can be saved for later on in the day, but studying and spell preparation would still be required. This could be extended to include spell empowering by spending more points than the minimum required. Maybe greater length or damage can be delivered by a spell if three or even four points are spent instead of the basic requirement for 2.
Now what stops my character from using all ten points on a high power spell? Wouldn't that immediately open 5th level spells to 1st level characters? Yes it would, but we could put a cap on this just like electric wiring has a cap on how much current can be conducted safely. Magical power could be expressed in points per action or some similar value. My character may have lots of points but can't channel them all in a single spell. Working on the previous example my character could have 3 points of "magic conductivity". Casting more than this may require some special roll to prevent the magic from failing. Failure would carry some dire consequences for my character so it might be something I'd think twice before committing to.
What I find interesting about exploring magic this way is how character attributes and skills link with the character's magic skill. When the magic quantums are small the magic system is much finer than when these quantums are big and the system is coarser. A plus one from an attribute relevant to magic use can be applied to tune magic in a much finer way. Does the bonus apply to the available points? To the usable points per action? We could even consider different attributes for different bonuses. Higher intelligence may add up more points, but physical constitution may contribute to "magic conductivity". In this way a player can't easily max out a stat and get all the benefits. It also opens up new dimensions along which the character can grow and having a small quantum gives more combinations and options; which makes my character less susceptible to being just like all other third level wizards. Do I put more less powerful spells? Do I put less, but more powerful ones? Do I up the power of each spell by investing more points?