Most of my list is around combat at the moment.
- Raise the baseline ability of characters to start heroic feeling even if the low HP and dice variability makes it dangerous. If a class is proficient in something they should at least be OK at it - like using two weapons, a ranged weapon, throwing a weapon, etc. I know there's a lot of noncombat stuff this applies to, but the most obvious cases are just being able to use the weapons a class is supposed to use.
- GREATLY improve maneuvers. Ideally, you should be able to make an attack and use a maneuver so you are not constantly weighing attempting a small action versus dealing the HP damage that a party is relying on. Either reduced damage to add an effect, or tie maneuvers to weapon damage to make weapon choice meaningful and allow a weapon use the maneuver as part of the typical damaging attack.
- Remove hyper-specialization in weapons. It is terrible, and even 5e pushes a fighter - a master of all weapons - to only ever use a small number of weapons. Specialization should be how to make a gish-y wizard (if anything), not how to make a fighter.
- Related to above, allow for carrying several powerful magical items without relying on your big 6. The best way this is done is simply by removing enhancement bonuses to magical weapons (or greatly reduce them). Make magic items to magical things and not abstract +X to roll things.
- Make mundane equipment more fantastic, and the choice matter more. This was done well later in Pathfinder and needs to continue - and be extended to armor and weapons more.
- De-emphasize stats, or at least the penalties. Pathfinder basically requires DEX, CON, and WIS for 90% of characters while CHA is relatively meaningless. The biggest issue is probably the negative modifiers combined with things like 2+INT skills, heavy armor, and save tracks. I don't have a great solution, but it's an old system that is showing its age.