Wizards and Sorcerers should have 9 levels of Arcane Spells.
Clerics and Druids should have 7 circles of Divine Spells.
Bards and Warlocks should have 5 mysteries of Occult Spells.
And they should all map to be equal at max-level for the power of a given spell. And also be largely, but not completely, separate from each other.
Sound insane? It is a bit. But c'mon, at least it would make the caster classes leveling more interesting and varied, and make room for stronger class and subclass abilities in the gap-levels between spell level increases for Bards, Clerics, Druids, and Warlocks.
Wizards and Sorcerers get access to a new level of spell every odd numbered level per usual. Clerics and Druids get a "Gap" level for 5th where they get cool Cleric and Druid themed stuff equivalent to extra attack or third level spellcasting. Same thing happens again at level 15 instead of getting 7th level spells. And, of course, Commensurately fewer spell slots.
Warlocks and bards, on the other hand, don't get 2nd level spells at 3rd, but at 5th their 2nd mystery spell slots are equal to 3rd level spells and they get something cool at level 3, instead. Same thing happens at level 9 and 11, then again at at 17.
All four of those classes get commensurately fewer spell slots compared to the wizard/sorcerer, with bard/warlock getting the least number of slots out of any 'full caster'.
And then the -actual- half-casters, Arcane Tricksters, Eldritch Knights, Paladins, Rangers, etc, follow the same sort of format. Tricksters and Knights get 4 levels of spellcasting, Paladins and Rangers only get 3 circles, but cooler tricks that better encapsulate Paladin/Ranger concepts when their circles skip.
L--S C M
1--1 1 1
3--2 2 1
5--3 2 2
7--4 3 3
9--5 4 3
11-6 5 3
13-7 6 4
15-8 6 4
17-9 7 5
I know what you're thinking. "That's too complex! It doesn't make sense!" to which I say "Okay. Cool. Do it anyway. Only way people come to understand is through either laborious discussion or simple demonstration."
They're not gonna do it, obviously, as it would mean more writing, more pages, more balancing between spells... and that's not something they want to do for their steamlined game... But it'd be so nice if they did...