I came to the same conclusion. Ban all other classes and build 4 classes just for the campaign setting. That is what I'm working on today:
- Shaman (dreamland focus, tailored spell list)
- Guardian (ranger-warlord mix)
- Warrior (fighter-protector mix)
- Beastkin (monk-half-beast mix)
No sub-classes attached. The power curve of the classes will be lower than regular 5e D&D. I don't plan to go beyond level 7-8 with this. I want an old school, less widgets, feel. From the DMG I will use: Proficiency Dice, Ability Check Proficiency (no skills). No Tools. Humans only. No backgrounds. Slow Natural Healing. A home-brew sanity rule. Looking at spell points instead of slots.
The setting is in a deep and vast forest. The technology level is lower than typical D&D level (no chain and plate armours). I'm aiming for a horror fantasy and gritty Grendel-like stories. The Gods are far removed and unreachable.