1. ) That said, if you have one party member being ludicrous more powerful than another, is due GM's fault. On most cases, wizards can go to shop and come back with ludicrous high amount of deadly spells that any non chaotic place would try to ban.If even on US is hard to get anti materiel rifles(some cases are considered DD) and a grenade launcher(always DD), why in a fantasy city getting a fireball scroll would be easy?I can picture every non chaotic major city outlawing spells like Finger of Death, Knock, Fire ball, etc.
2.) On pathfinder kingmaker, the scrolls available on your barony shop is tied to his "arcane" rank and before you can raise arcane, you need to raise your domain divine.
3.) I will re write your sentence "casters are op" to "wizards when they can get a complete spellbook with all spells in existance and have time to rest after every encounter are OP"
4.) Warlocks, should get only spells that makes sense that his patron will teach him. A warlock who has Raven Queen as Patron for eg, should't cast wall of light. Same with a sorcerer, a sorcerer with golden dragon bloodline casting frost spells 24/7 would make no sense.
I have edited your post a bit to make a reply easier
Your ideas are good in principle but you have to implement this in a computer game.
1.) In BG2 the cowled wizards would come if you use magic in the city, at least until you pay them.
It was not a big problem as there were no difficult encounters outdoors in the city and the money was easy to get.
Many players would get upset if you cannot find some typical mage spells.
If you cannot find some scrolls, classes who learn new spells at lv up would become more powerful compared to wizards unless 4.)
2.) In PK you ruled over this city. You will probably not rule a city in BG3.
You get camp followers however. Maybe your camp merchant gets new items only when you make a deal with some groups in the game world and some groups are exclusive. For example you could help either the city guards or the thief guild and each one gives different items.
3.) It will be hard to prevent players from resting in a computer game, unless you add a time limit.
Maybe some quests progress automatically when you rest.
For example you get a quest that bandits want to attack a town. If you go there at once you find the bandits in their camp and deal with them before they attack the town. If you rest once the will attack the town when you arrive. If you rest twice or more they have occupied the town and killed or imprisoned most villagers.
This would lead to players resting every time before accepting a quest,
Unless you put a hard timer on every quest. But this would mean that fast players have to wait forever until an event starts while other players miss stuff without knowing why.
4.) You could progam that each magic subclass has only access to a limited amount of spells of their class.
But the way I understand 5E, subclasses add abilities to the base class, they do not remove stuff in general.
I like these ideas and a good GM could implement them in his game,
but I do not expect to find this stuff in a computer game.
A GM has to make a game interesting for a handful players in front of him.
A computer game dev has to make the game interesting for millions of people. I guess there are more casual players or min maxers than dedicated role players who consider role playing more importent than maximizing your power from a game mechanics point of view.