I think if you are going to expand, you need to start with a different core 4. Namely, fighter, ranger, wizard, druid. Ranger with no spells, more stealth. Visualize those in a circle.
Then slot either the cleric or the paladin between the fighter and druid. Slot the thief between the ranger and wizard, expanding a little on the thief magic user scroll use to give them the wizard spells the ranger used to have. Or put an old-school bard there if you prefer, though I don't like the "bard" name on that class, as I prefer it to be an off-shoot of the druid.
There are other ways to do it, of course, including different visualizations besides a circle. For old-school, I'd prefer to keep it down to no more than six classes per race, with the above being the human options. So there might be 10-12 classes, but they 4-6 that any given race can use are on some kind of clear circle or other simple diagram.
Point being, to expand is to change the core 4 in some way. Expansion works best when it takes that into account.