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.