My ideas would require a new expansion, but I would be competley fine with that as long as we can keep characters and games seperate.
Major Changes
New Act
More Act IV quests
Act Zero Diablo 1 Dungeon because why not?
Minor Changes
Balance changes where needed
A 4th difficulty that is all the same level
No level requirements for skills and no prereq requirements.
Stackable gems/runes
Instance loot for Runes
Dedicated Charm slots
Skill bar or some easy way to hotkey abilities.
Level 100 cap because I am OCD
Tone down immunities
But don't hold your breath. Based on blizzards track record with WC3R, I wouldn't expect much