Wow, in one thread? You are a brave... person.
1) Proper party control/managment. Let us control all party members at once, not only the currently selected one who the others are "chained" to... proper selection and formations.
2) Portraits should be right clickable to open inventory, and we should be able to cast/use items on portraits.
3) ... general, proper inventory management.
4) Ammunition and other resources.
5) Day/night cycle, fog of war.
6) Overworld map with random encounters and respawning monsters/randomized encounters on area maps.
7) Custom hirelings/mercenaries to replace party members.
8) Proper camping system with characters/hirelings filling certain roles, like hunting for rations, guard duty and crafting.
9) Proper implementation of 5e combat. Dial down on the surface effects and bonus actions. Stick to MM.
10) Keep classes distinct.
11) Proper 5e skill rules imp mentation. Use the D&D loot tables.
12) More detailed, "realistic" handling of gear (no floating swords!)
13) ...and so on, and so on, and so on.