Last Guy in the Airlock
This is pretty much exactly what Shadow of the Demon Lord does. If this is what you want, I would suggest you take a look!I'd rework them thusly:
1. I'd have only four "core" classes. These would be the foundation that the rest of the character would be built on, and would set things like Saving Throws, proficiencies, and spell slots. (These names are place-holders, I'm sure they will come up with more iconic/traditional ones.)
2. Then I'd have dozens of interchangeable subclasses. You would pick your subclass at 1st level, and apply it to the framework of your choice. This would set class-specific things like sneak attack, spell lists, turning undead, etc. A Warrior with the Barbarian subclass would play much like a Berserker, while a Priest with the Barbarian subclass would play a lot like a Wild Magic barbarian, for example.
- Warrior (martial-focused)
- Sneak (skill-focused)
- Mage (full caster)
- Priest (half caster)
It's a bit of a call-back to the "kits" of old.
Four classes, as you describe, with abilities that kick in at levels 1, 2, 5, and 8.
Race/ancestry, with abilities that kick in at level 1 and 4.
"Expert paths", with abilities at 3, 6, 9 (assassin, paladin, druid, ranger, warlock, etc.)
"Master paths" with abilities at levels 7 and 10. This can be a second Expert path, or any of a wide range of niches (blade, chronomancer, diplomat, enchanter, necromancer, zealot, etc.)
It's a great system for mix-and-matching kits to create unique characters, no dead levels (as you see) and every character develops along the same progression (which some won't like, of course).