As a longtime player I have a few ideas that I believe would improve the game.
Simulation: I cant tell how much time I have used to try and figure what diffrent skill and devil fruit do and even then, I am often wrong. My solution to this problem is sort of a tutorial feature that will display what a skill or fruit actually do, an example would be haki Zoro's skill: inferno ghost chop: a simulated haki Zoro would stand against full field of enemies, use his skill damaging all in the line in front of him and buffing himself with crit and attack. I acknowledge this might be a bit overkill if the descriptions of skills and fruit were made more easily understandable and would it take considerable amount of work to make, but its unbelievably annoying to equip a devil fruit and find thats its proc is not a buff to your original attack but actually replaces it entirely with a new one. (I am looking at you heavy fruit)
Homestead: There are so many crewmembers available, but so few of them are actually viable to use late-game or even midgame. My idea is a feature that allows the use of all available crewmembers by putting them to work: bounty hunting, building, exploring etc.
The idea is to put teams together to handle diffrent tasks and based on their stats (str, agi, int) and level how long a task would take and/or how probable success is. There could even be a theme bonus if you put certain members on a team, like capt. Buggy, Cabaji and Mohji or the CP9 members, giving some incentive to "collect" crew that you otherwise wont use.
As you complete task your homestead grows and more buildings and tasks become available. A few ideas of buildings would be an opal shop (opals are basically useless when you have all the treasure buffs you want), favor exchange where you at the cost of favor can trade one resource for another like stone to agate or coral to pearl (because the only place besides training you can use favor is advancing!) and a laboratory that increases the effect of your research based on its level.
Thanks for your time, I hope my ideas will be taken into consideration and if not, at least inspire other ideas.