1. Mandom from 7th part, his iconic ability should be to go back 6 seconds at like 4th tier, but i don't know if its possible to code because it sounds hard.
2. Weather Report from 6th part, he has unlimited possibilities and definitely should have 5th(requiem) tier, some ideas for the special abilities i thought of creating raining animation with pouring down acid rain and then later maybe upgraded to raining knives, final tier should have an ability that the closer you get to him, more damage you will receive (oxygen reduction, emporio used against pucci), and maybe early on he should have ranged attacks like horizontal tornadoes.
3.D4C from 7th part, i think it should be 5th tier also, of course it should have ability to clone and higher the level the more clones and for longer they would last, the clone should just shoot with a pistol, i didnt wanna include broken abilites but when i read golden experience ability of death loop to one shoot even bosses i think you can then include ability for the love train to be immortal, as he reaches 0hp he could just rise from the ground full hp, of course total immortality would be to broken so it should have some kind of catch( maybe he could die only if he was in mid air, as he had to be between objects to come as a new man, or make that ability on a cooldown) also it should definitely make tusk act 4 to be the only one able to kill him without any catches, and last one thing, love train = increased luck.
4.Death 13 from part 3, a stand that could make enemies sleep, the stand should use scythe and later throw them as projectiles, then a special ability that would change world colors(indicating dream world) something similiar like the worlds times stop, but for much longer it should for example make enemies smaller slower reduce their defense by 50%, have random ♥♥♥♥♥ that attack those enemies or maybe just buff the stand itself(make it really fast and long range). In multiplayer other stand users should get their stands disabled.
5. Highway star from part 4, have ranged ability to shoot homing footsteps, later make that it leaches to enemy and drains their health to you.
6. Green day from 5th part, shoot poisonous smokes, "Autonomous Limb Control" from the wiki, perhaps make it regenerate rapidly for a few seconds instead of insta heal so it wont be repetitive with golden experience, and lastly make you emit lots of poisonous gas as you move that dissapears after like 10 seconds.
7. Heaven's Door from part 3, perhaps to use it as a tool that you can choose to upgrade yourself (have specific amount of skills points that you can put into like health, damage, luck, speed and so on and unlock extra points to put with extra tier, so to sum up make it something like an rpg).
8. Lastly, WOU from 8th part, it is kinda overpowered and one of my favorite stands from the whole franchise, but the problem is its kinda hard to make it work, except maybe make it an automatic stand and to use the previously mentioned ability for weather report, that the closer something gets to the user the more damage it takes and more debuffs it gets, i'm kinda bias and think it should be requiem tier as the last upgrade should make that whenever something really touches you (physically, don't count the enemy projectiles) it would just instantly die.

Also there could be more content for Hamon class, more armor, make pillarmen bosses, and ofc how couldnt you include Gyro's steel ball, it should be unlockable after hardmode boomerang type thing.