As a huge fan of the spell memorization type of magic I did a lot of thinking on it. One of my first concepts was the idea of "pits"
The idea is that spell slots are divided into compartments that are filled when you memorize spells,
a cantrip takes up 1 pit , a first L spell takes up 4 a second L spell takes up 6 a third L spell takes up 8 and so on by two. This also explains what can be substituted for what.This a first L spell can be turned into 4 cantrips and a third L spell can be turned into 2 first.
This also led me to create 1/2 L spells or as I like to call them petty spells, they fill in two pits, but they are not on the spells allowed list. In the lore of my world they are not popular and there are not that many of them. The only one that caught on with my players is Shout, it allows you to yell super loud for L rounds.
Another aspect of spell memorization that is mostly of use in RPing situations is I attached emotions to the spell schools that you must place your mind into in order to memorize spells of that school. All mage carry around a book that is filled with very short stories that enable you to create the emotional state to memorize.
For example , you would read a story about your birthday party when you were young, it would lead you to wonder what was in the brightly covered box in front of you. This would evoke the curiosity necessary to memorize a divination spell. I know, I know its both kinda cool and mostly useless.