add more mobs some boss fights and stuff like that there are no enemies after you become op
weapons being able to consume devil fruits ik this is very op so i recommend them to only work on iron weapons and under with and if possible make it so that if the weapon has mending it doens't work so after the weapon breaks it can be reset to the world. ik making abilities and stuff for every df and weapon is difficult but i recommend the weapons can only consume zoan and maybe mythical zoan fruits this could possibly make people use swords guns and maybe even the weather class more.
some quests that are given from npc example like one for pirates one for marines and bounty hunters and so on and maybe some kind of story mode if possible but if not just some quests to make the game more fun.
making it so you cant get doriki from endermen would be good because of how fast you can get doriki and it ruins the fun.
adding more abilities to weapons and other fighting styles cause currently as we know fruits is probably the only way you become strong and haki so making it so there is maybe a powering system for weapons mastery and so on to make it so weapons can be on par with actual fruits with cool strong and better skills so people use them more.
make every fighting style on par with swords man ik this sounds a bit biased but like most skills dont do a lot like the doctor i mean yes its a doctor but no one uses it so if maybe there was a way to make it good for every day use like some kind of way to use it in pvp which is kinda the only thing needed in the one piece mod and snipers and weather stick user forgot the name but they kinda mid i mean maybe some better attacks that did something and not just maybe stun for a second or do no damage.
last suggestion make sea stone a bit better i mean they do nothing if you could make them like unbreakable to devil fruit abilities and the users of df's and maybe make the bullets do something to user cause those bullets should hurt a lot even for the strongest of enemies this way people use the sniper.