The real issue like I said before is the current statblocks favor casters and massive sacks of HPs.
Like the Priest (caster) shares CR 2 with
- Bandit Captian (~25% more HP and ~4 more AC)
- Berserker (~75% more HP)
- Cult Fanatic (caster)
- Druid (caster)
- Mage Apprentice (caster)
And thats just Cr2. Martials and half casters have fewer options. So the Players will want to be compensated.
Its not a bad idea. However for it to work, you have to be okay with when the Wizard NPC as PC gets Fireball, the Fighter and Monk NPCPC has 25 AC, the Barbarian has 100 HP and 5 resistances, and the Rogue, Paladin, and Ranger deal a sack full of damage per hit.
Its a cool idea. I'd love to try that game. But the numbers will have to inflate hard.