An open discussion question for polite musings.
Currently, 5e D&D has full casters (i.e., wizard, bard, druid, cleric, sorcerer), half-casters (i.e., artificer, paladin, ranger), and even 1/3 or one-third (e.g., Eldritch Knight, Arcane Trickster, etc.). Absent within this quarter-based schema is a 3/4 or three-quarters caster. Is there a reason that 5e either chose not to design a 3/4 caster and is there room in the game for such a progression? Furthermore, would any of the existing classes have been better off as 3/4 casters than their current spell progressions?
So here's how I approach the issue- we have 13 classes.
Of those, five (more than 1/3) of them are full casters. Wizard, Bard, Druid, Cleric, Sorcerer.
Another three are 1/2 casters. Artificer, Paladin, Ranger.
Another one uses a different mechanism, but is arguably a fully caster. Warlock.
So 9/13 classes (70%) are already brimming with spells.
The four remaining classes are Barbarian, Fighter, Rogue, and Monk. Those already have two options for 1/3 casters. Maybe more depending on how you classify some of the Monk subclasses.
In short, the issue isn't the dearth of design space for casters; it's the absence of real non-caster options.
Now, let's put that aside. Assume you want more spellcasters and more variety of ... bewitching? Enspelling? Blasting? Anyway, this is the issue you come up with-
The design space between half-caster and full caster is already very limited. I would say that while there is sufficient differentiation between half-casting (in terms of spell slots and maximum spell level) to be worth it, the difference between a full and a 3/4 (or 2/3) caster just isn't there in 5e because there aren't enough tradeoffs elsewhere.
So, I would look at this in a slightly different way; in order to introduce differentiation, I would not have 3/4 casters. Instead, I would promote variety through one of two methods:
A. Completely differentiated spell lists. The primary issue of "samey-ness" between spellcasters is largely due to overlapping spell lists. If you want spellcaster to be different, you need to more strictly patrol the boundaries of their spell lists (but this is often unpopular with players).
B. Mechanics. The simplest way to make the spellcasters play differently is to change them up mechanically. The two best examples of this are the Paladin and the Warlock. The Paladin does this in the simplest way possible- by using spell slots to power a different ability (smite). For many players, Paladins seem more martial than half-castery because they are using their spellslots for a specified purpose, not to cast spells. The more involved method is to create classes like the Warlock, which have different underlying mechanics that allow for differentiation.