I've lowkey been considering a full JJBA roster for some time, and it's hella sweet to see this here! Nonetheless, I may as well try my own takw on something unique - namely, what a JJBA platfighter similar conceptually to something like Smash could look like. I'm going to keep a similar "style" for characters, though.
Part 1: Phantom Blood
Jonathan Joestar (Hamon) - It's Jonathan wooo
Will A. Zeppeli (Hamon) - Hey, baby!
Robert E. O. Speedwagon (Ogre Street) - I think this style works pretty well for him. Bringing in either Tattoo or the Kempo Fighter works pretty well, but having the focus be on Speedwagon is cool. Also, I think his Part 2 design would make a lot of sense as an alt.
Bruford & Tarkus (77 Rings) - I picture these two as being different to Speedwagon in that they actively work together for a lot of their attacks, as opposed to just bringing in assists. Working together would result in them having a much more unique moveset, and I think their style name makes a lot of sense too.
Dio Brando (Vampirism) - It sure is Dio Brando, the invader. He has his whole array of vampiric abilities here.
Part 2: Battle Tendency
Joseph Joestar (Hamon) - Next you'll say, "Well obviously Joseph is in the game!"
Caesar Zeppeli (Hamon) - SHIIIIIIIIIIIIIIIIZAAAAAAAAAAAAAAAAAAAAAAAAA
Lisa Lisa (Hamon) - Hey, she's cool. Her scarf gives her a really unique vibe, all things considered.
Stroheim (German Science) - I think it'd be cool if he's able to sort of "mode switch" between either his minigun form or his UV form, but I'm not 100% sure how actually useful that'd be. Aside from this, having access to the tons of machine guns we see him with at the end of the anime would be awesome too.
Straizo (Vampirism/Hamon) - This probably feels like a really weird pick, but I think Straizo having a unique take on Hamon due to his vampirism where it actively damages him to use it - as well as finally introducing more than one Vampirism character - would be awesome.
Wamuu (Pillar Man) - Insert Darude Sandstorm MIDI
Esidisi (Pillar Man) - I think his blood manipulation makes him a solid pick here. That's about it to be honest.
Kars (Pillar Man) - Kars is based here purely on his normal form, meaning he mostly uses his bone blades and Light Mode in combat. I considered having him transform into his ultimate form, but given the sheer amount of differences in moveset, it made sense to me to make him seperate.
Ultimate Kars (Pillar Man/Ultimate Lifeform) - This form is moreso based on Kars as the Ultimate Lifeform. Not much to say here.
Part 3: Stardust Crusaders
Jotaro Kujo (Stand) - Did you really expect him not to be here? I'd say in this case his Part 4 and Part 6 appearances are alts with unique voicelines and models. Unlike, say, Kars, it doesn't make as much sense for him to be completely unique for each one. I wonder whether he could also have a Jouta alt...
Joseph Joestar [Part 3] (Stand/Hamon) - His taunt has him break a camera. Also, his Part 4 appearance is an alt of his too.
Muhammad Avdol (Stand) - YES, HE IS.
Noriaki Kakyoin (Stand) - The archetypical mid-range character.
Jean-Pierre Polnareff (Stand) - He'll build Polnareffland one day, I swear. Also, he has a Part 5 alt (minus the wheelchair.) Not sure why he never gets this one.
Iggy (Stand) - Say what you will, but Iggy being basically Kragg in being able to create platforms of sand could be a cool way to represent his ability.
Hol Horse (Stand) - Similar to in Heritage for the Future, Hol Horse fights alongside J. Geil here - though mainly just Hanged Man. It's mostly to make him unique from others that use guns or gun-like weaponry.
N'Doul (Stand) - I think he's a solid alternative to Mariah. Yes, he might not have the funny breast move, but he's a cool character with a cool Stand - and a lot of potential given he typically stays still when fighting.
Pet Shop (Stand) - He might not be the most important character in the part, or even the most popular, but he's still a solid choice with plenty of potential.
Vanilla Ice (Stand/Vampirism) - Very few actually vampiric abilities. Also, I'd say there needs to be a cooldown on how much he can use Cream.
DIO (Stand/Vampirism) - Similar to DIO in Stardust Crusaders, he barely actually uses his vampiric abilities. However, his vampirism does mean he takes slightly more damage than most from Hamon-based attacks.
Part 4: Diamond is Unbreakable
Josuke Higashikata [Part 4] (Stand) - Sorry, did you just say something about his hair?
Okuyasu Nijimura (Stand) - OI JOSUKE, I'M IN THE ROSTER DESPITE BEING AN OBVIOUS PICK, AIN'T THAT WACKY?
Koichi Hirose (Stand) - I think the three-stage evolution could work here, but a solid alternative could be him already being at Act III and being able to reverse his evolutions.
Rohan Kishibe (Stand) - You can't just tease someone without permission!
Yukako Yamagishi (Stand) - Hair. Yes, she is conceptually similar to Bruford.
Akira Otoishi (Stand) - I think he should use his guitar to batter people to death.
Mikitaka Hazekura (Stand/Magellanic) - The name comes from his origin. Honestly, he's just a good fit. Shapeshifting is cool.
Yuya Fungami (Stand) - Hear me out here - he is a massively underrated pick that could be really fun to see. His Stand having the ability to drain an opponent's health to increase his own could be great.
Yoshikage Kira (Stand) - Yes, only one. I think it makes sense to keep him at his base form and then maybe have Yoshihiro come out to cause Bites the Dust as a super of some kind, then give him the Kosaku Kawajiri look as an alt. Maybe his Part 8 form could be an alt too, or am I pushing it...
Part 5: Vento Aureo
Giorno Giovanna (Stand) - Breaking: gay child has a dream, takes over entire country in one week
Bruno Bucciarati (Stand) - And for his neutral special, he births twins
Leone Abbacchio (Stand) - Okay, so my theory here is that Abbacchio's Stand Mode would act uniquely in that it essentially allows him to buffer his own attacks to provide a multihit effect, or can be buffered further back in order to act as a trapping tool but loses the multihit as a result. He's a strong guy anyways, being an ex-police officer, so I think it's not unreasonable to imagine him kicking the **** out of someone like Kira or Pucci.
Guido Mista (Stand) - Look at his position. Jokes aside, his Sex Pistols gives him a really unique vibe compared to other "gun users" such as F.F. or Hol Horse, given his ability to manipulate bullet movement and his limited ammunition.
Narancia Ghirga (Stand) - A long-range Stand user is always cool to have, but the idea of Narancia just pulling a knife out and going ape**** up close feels fitting too.
Pannacotta Fugo (Stand) - Poison is cool. That's about it really. Oh, and Purple Haze ripping people to shreds would be fun too.
Trish Una (Stand) - Softening could give her the ability to remove hitstun, which seems like a really unique mechanic that she could have honestly.
Ghiaccio (Stand) - IT'S DEATH IN VENEZIA BECAUSE IT'S SET IN "VENICE"
Risotto Nero (Stand) - Yeah he kinda had to be here it made too much sense not to be
Cioccolata & Secco (Stand/Pet) - I think Cioccolata would be fun even on his own, given his medical expertise and pretty interesting Stand ability. Adding Secco to the mix gives him even more tools! Also, is it just me who prefers Green Day's design in the PS2 game?
Diavolo & Vinegar Doppio (Stand/Personality) - So whilst I originally considered Diavolo being pretty basic, I think it'd be really cool for them to have a kind of Mode Switch where you can play as Doppio (using King Crimson's arms and Epitaph), or just outright Diavolo (using a more traditional King Crimson.) It'd be awesome!
Part 6: Stone Ocean
Jolyne Kujo (Stand) - But what did the guard see you do, Jolyne?
Ermes Costello (Stand) - SHE ISN'T A ROCK HUMAN, STFU
Emporio Alnino (Stand) - I think Emporio would be an interesting pick to see here, and he's actually the reason I ended up going with Fungami as opposed to Terunosuke. Having an "object summoner" archetype seems like it could be really interesting, and I think it'd make him undeniably unique. Plus, well, we all know what his most powerful move should be.
Foo Fighters (Stand) - Having her move around in unique ways to represent her unique nature could be a lot of fun. I also like to imagine she'd be capable of healing herself somewhat, maybe? Though, a water meter feels a bit much.
Weather Report (Stand) - Weather Report has the ability to control the weather
Narciso Anasui (Stand) - Say what you will, but I think Diver Down has some solid potential.
Kenzou (Feng Shui) - He feels like a solid minor antag pick, and his Feng Shui mode allows other players to use his Dragon's Dream to their advantage. He mostly fights with karate-type moves, and maybe he could have Survivor helping him out too. He's also a solid rep of the Maximum Security arc, which is cool.
Rikiel (Rods) - Yes, Rods are a unique style for this guy. I think it fits - he's unique in that the Rods can slow his opponents down and the like. I'd have them constantly visible, though, purely for balancing purposes.
Enrico Pucci (Stand) - Similarly to Kira, Pucci's moveset is based moreso on his later iteration. Imagine him being more like New Moon Pucci. He does still use Whitesnake at the start, but he can evolve to C-Moon and Made in Heaven with the help of the Green Baby.
Part 7: Steel Ball Run
Johnny Joestar (Mounted/Stand) - Again, Stand evolution, but with an ammo meter and with Mounted style. He's not exactly approachable.
Gyro Zeppeli (Mounted/Stand) - My sixth lesson, Johnny, is how to get some bi-
Diego Brando (Mounted/Stand) - Scary Monsters is solid here, and honestly I think it'd be nice to see him partially turn his horse into a dinosaur too.
Hot Pants (Mounted/Stand) - CREAM STARTER IS UNDERRATED
Mountain Tim (Mounted/Stand) - Okay so, he might not seem exactly obvious but I think his Oh! Lonesome Me would make for an incredibly interesting take on a mid-range grappler archetype.
Soundman (Stand) - Fastest run speed in the game, need I say more
Blackmore (Stand) - He's popular for a reason. Also, it always rains when you fight him.
Funny Valentine (Stand) - Who shot Johnny Joestar? No ****ing idea
Part 8: JoJolion
Josuke Higashikata [Part 8] (Stand) - Can he please have an updated moveset? Oh, and a Josefumi alt. That'd be cool too.
Yasuho Hirose (Stand) - Paisley Park could potentially give her additional levity when dodging, and being able to come out of her shadow could be fun too.
Rai Mamezuku (Stand) - CROSSBOW. NEUTRAL. SPECIAL.
Joshu Higashikata (Stand) - Honestly this guy gets way too much crap from people, I'm pretty sure it's purely because of his haircut
Jobin Higashikata (Stand) - I think it'd be fun if he has a beetle in his moveset. Like, at least one. Maybe as a heat bomb or something.
Yotsuyu Yagiyama (Stand/Rock Human) - Originally considered Damo here, but he doesn't feel like he'd lend well unless he were with Hato, and that'd make no sense. Nonetheless, Yotsuyu does still have an important role - and honestly feels like he'd be a solid trapper character.
Urban Guerrilla & Doremifasolati Do (Stand/Rock Human) - Honestly, this feels like a great inclusion. Similar concept to Secco in the whole "underground" thing, but given Urban has to get inside Doremifasolati Do to use the ability, it's more of a traversal or surprise tool. Brain Storm is also a cool Stand.
Tooru (Stand/Rock Human) - I originally considered Akefu here, but I think having Akefu as an appearance for Wonder of U could be great.
Spin-Off Media
Yoma Hashimoto (Avatar of Hermes) - I think he's a solid Thus Spoke rep, and his unique nature - not having a Stand but instead being focused on fitness - would give him an overall unique vibe.
Massimo Volpe (Stand) - I think a Stand focused almost completely on either buffing or debuffing opponents gives this guy a solid shot. Also, based PHF rep.
Araki's Guests
Ikuro Hashizawa (Armed Phenomenon) - He's in ASB, it's basically that.
Irene Rapona (Gorgeous) - I think her assassin techniques and use of makeup would make her a formidable close-range opponent with a high skill ceiling, but someone who's also approachable for newcomers due to her relative simplicity. Also, her old lady disguise would be a solid inclusion as an alt.
This is essentially a fever dream, but I wanted to make sure each part got at least reasonable representation. This'd never get made, of course. I'd like to have seen some others get into here (Proscuitto and Pesci was a difficult omission) but this feels fairly good all-round.