1) Improved demarcation of class abilities - eg. scrolls only being usable by classes the spell list appears on, including Wizards only learning spells from their list; make disengage and hide an Action so that Rogue's Cunning Action feature feels significant
This, so much this. I made an account just to come on here and post this and agree. Classes MUST be more unique. Every class needs to have its clear limitations. Allowing everyone to use the same spells (even if they are barely a magic user) is not good. That was one of my biggest issues with DOS1 & 2 - classes felt generic.
Dos is classless, so that was fine there.
In BG3 however I have to agree.
5e does allow everyone to use scrolls though.
But maybe if the usage was limited by proficiency a specific skill like Arcana and Religion, for arcane and divine magics respectively, it would improve the character uniqueness and flavour, giving even more importance to the background choice.
Edit: maybe if not proficient you could use with a check based on spell level and failure could be a little worse than nothing happening? Just another idea.