So the starting roster would be 16 characters. Then there'd be 4 DLC seasons composed of 8 characters each, for a grand total of 48 characters, because 48 is just a really nice number for fighting game rosters. I could stick most of the roster in one of 4 categories: Protags, villains, important side characters, and ensemble darkhorses. This last category is important to take in consideration for some of the character choices further down the line. I also tried to avoid more than one character spot going to the same character as much as possible, but there IS one exception that you will see pretty soon. But basically this means that you won't be seeing, say, Old Joseph or Part 4 Jotaro.
Base Roster:
Part 1: Jonathan Joestar, Dio Brando
Part 2: Joseph Joestar, Kars
Part 3: Jotaro Kujo, DIO
Part 4: Josuke Higashikata, Yoshikage Kira
Part 5: Giorno Giovanna, Diavolo
Part 6: Jolyne Cujoh, Enrico Pucci
Part 7: Johnny Joestar, Funny Valentine
Part 8: The other Josuke (who fans refer to as Gappy), Jobin Higashikata
Season 1:
Part 1: Robert E. O. Speedwagon
Part 2: Caesar A. Zeppeli
Part 3: Noriaki Kakyoin
Part 4: Okuyasu Nijimura
Part 5: Bruno Bucciarati
Part 6: Ermes Costello
Part 7: Gyro Zeppeli
Part 8: Yasuho Hirose
Season 2:
Part 1: Will A. Zeppeli
Part 2: Lisa Lisa
Part 3: Avdol
Part 4: Koichi Hirose
Part 5: Guido Mista
Part 6: Narciso Anasui
Part 7: Diego Brando
Part 8: Rai Mamezuku
Season 3:
Part 1: Tarkus
Part 2: Rudol von Stroheim
Part 3: Jean-Pierre Polnareff
Part 4: Rohan Kishibe
Part 5: Narancia
Part 6: Weather Report
Part 7: Ringo Roadagain
Part 8: Joshu Higashikata (joke character)
Season 4:
Part 1: Bruford
Part 2: Wamuu (yes, unfortunately i had to leave out esidisi for the sake of german science)
Part 3: Hol Horse
Part 4: Akira Otoishi
Part 5: Trish Una
Part 6: Foo Fighters
Part 7: Hot Pants
Part 8: Tamaki Damo
So, the final roster would be this:
Part 1: Jonathan Joestar, Robert E. O. Speedwagon, Will A. Zeppeli, Tarkus, Bruford, Dio Brando
Part 2: Joseph Joestar, Caesar A. Zeppeli, Lisa Lisa, Rudol von Stroheim, Wamuu, Kars
Part 3: Jotaro Kujo, Noriaki Kakyoin, Avdol, Jean-Pierre Polnareff, Hol Horse, DIO
Part 4: Josuke Higashikata, Okuyasu Nijimura, Koichi Hirose, Rohan Kishibe, Akira Otoishi, Yoshikage Kira
Part 5: Giorno Giovanna, Bruno Bucciarati, Guido Mista, Narancia, Trish Una, Diavolo
Part 6: Jolyne Cujoh, Ermes Costello, Narciso Anasui, Weather Report, Foo Fighters, Enrico Pucci
Part 7: Johnny Joestar, Gyro Zeppeli, Diego Brando, Ringo Roadagain, Hot Pants, Funny Valentine
Part 8: Part 8 Josuke/Gappy, Yasuho Hirose, Rai Mamezuku, Joshu Higashikata, Tamaki Damo, Jobin Higashikata
Aaaaand that's it. I know there are some omissions (4 characters I really wanted to include were the aforementioned Esidisi, Yukako, Fugo and Abbacchio), but I hope I managed to deliver a satisfying roster nonetheless.