A good number of JJBA fans are also fans of other Shonen Jump properties.
A lot of long time fans are fans of Fist of the North Star, owing to the fact that JoJo was hugely influenced by it in its early years. Both series feature manly men doing manly things and insane amounts of over the top the violence. It helps that the first three installments' designs were inspired by Tetsuo Hara's artstyle.
It is very common to see fans of JJBA who are also fans of Gintama. Considering how both series run in Shonen Jump and parody other Shonen Jump titles that ran at the same time, fans of one series are very likely to find appeal in the other. Especially made better when Tomokazu Sugita who voices Gintoki would eventually voice Joseph.
Thanks to Diamond is Unbreakable there has been an increase in the amount of JJBA fans who are also fans of YuYu Hakusho which ran in Shonen Jump while Diamond is Unbreakable was publishing and featured similar themes. Years down the line Diamond is Unbreakable would also produce an example of this trope with Persona, with the fourth game in particular having the most overlap because of their similarities as noted below.
Many JJBA fans are also fans of Assassination Classroom which has a similar tone to JJBA and the author was even inspired by the JJBA franchise. Due to the similar tones that combine over the top humor, characters, and action with some serious dramatic moments some have even seen Assassination Classroom as a Spiritual Successor to JJBA now that JJBA no longer runs in Shonen Jump. It helps that Mr. Matsui, the creator, actually did a few cooking videos with Hirohiko Araki, although they were on a Japanese-only site that has since pulled them down. Only the promos (and a brief comic by Matsui about meeting his idol) remain.
A lot of Castlevania fans are also Jojo fans, due to both series being a Generational Saga against a powerful vampire that came out at around the same timenote Castlevania released in 1986, and Phantom Blood in 1987. The Castlevania games themselves are filled with many overt homages to Jojo, further strengthening the camaraderie between the fanbases.
For more than obvious reasons, the series attracts many fans of Guilty Gear, another rock-influenced franchise based around weirdos named after musicians kicking the hell out of each other in absurd and over-the-top way.
Over the past few years, music mash-ups involving the opening themes have become popular among the fandom... usually mixing the openings with music from or related to Love Live!. As such there are numerous fans of JoJo's Bizarre Adventure who are also fans of Love Live. Some or half of them even joked that both of them shared the same universe, as Sunshine had (coincidental) references and "callbacks" to the manga.
Due to the anime using real world songs for the endings, this pops up a lot on YouTube. It is a great cross-pollination of JoJo fans who haven't heard of bands with fans of certain bands more than happy to have new fans. The process usually goes- A new ED is released. Fans of JoJo bombard the song on YouTube. Fans of the song ask, "What the hell is JoJo?". Anime fans spew memes and explain what JoJo is. Band fans get into JoJo, while telling anime fans "This song is great but listen to some other songs by this artist." Anime fans listen to other songs and gain more respect for the bands. If anime fans don't care for the song or the band, it's okay! The sheer influx of anime fans checking out the original song send the view count to incredible levels.
The anime has created this in regards to the franchise and music artists and their fans, due to the anime getting the ending themes from various bands. JoJo fans are happy to have a new group of people to explain what the franchise is about and gaining new followers. Music artists and their fans are happy to have a new group of people to introduce music to and give friendly recommendations that "this band had other songs you know". Reached it's zenith when Savage Garden released a Greatest Hits album and one of the album covers was of Josuke Higashikata from Part 4 as a way to thank the anime fans for beings such huge fans of the song used as the ending for the Part 4 anime.
It's not uncommon to see fans of JJBA who are also fans of The Simpsons since both are long-running series that have created countless memes over the years. It helps that the Joey Jo-Jo meme from The Simpsons also crossed over into the JJBA fandom.
It isn't too hard to find overlap between the Homestuck and JoJo fandoms, given the amount of crossover memes the fandoms produce. It helps that both tend to suffer It's Popular, Now It Sucks! thanks to both fandoms being rather large and vocal. Some have even called JoJo "the Homestuck of anime".
While some fans have called the latter series a ripoff in the past, JoJo fans and Persona fans have grown increasingly cordial towards one another. Fans of both series find common ground in how both JoJo and Persona involve the use of physical manifestations of their spirit, yet fans also acknowledge that both series take the concept in their own distinctive ways. In particular, fans love to point out the similarities between Persona 4 and Diamond is Unbreakable involving several youths hunting down a serial killer trying to keep his identity hidden, and Persona 5 and Vento Aureo involving a thief joining a gang of criminals who want to improve society.
Many JoJo fans are also fans of Touhou Project. This is mostly because the series' creator has admitted himself of being a JoJo fanboy, and the series itself often makes reference to it.
Fans of Yu-Gi-Oh! and its sequels love comparing it to JoJo. It's easy to see why thanks to both series taking their respective subjects very seriously while acknowledging their over-the-top antics, memetic hand and body poses, and Ham-to-Ham Combat. The storytelling of both series is also similar in execution, split into separate parts that each following a new protagonist contrasting the last, but the same overarching theme of overcoming fate and friendship. There is also a theory that Kazuki Takahashi was inspired to make a card game by the D'arby scenes in Stardust Crusaders; he is at least on record as admitting to have been inspired by the darkness in JoJo and the defeat of Telence is very similar to how Yugi would defeat Pegasus. The real kickers for many though are twofold; a) Sartorius/Saiou from Yu-Gi-Oh! GX, who uses a tarot-themed deck that includes a monster based on The World who has the effect to skip a player's turn, and who is voiced by Takehito Koyasu himself before his role as Dio. Needless to say, the fans and Koyasu himself when he reprised the role for Yu-Gi-Oh! Duel Links, have had an absolute field day with this. b) The protagonists of both franchises' fifth series (Giorno Giovanna from Golden Wind and Yuya Sakaki Yu-Gi-Oh! ARC-V) are both voiced by Kensho Ono...and in the dub, Giorno and Part 4's Yuya Fungami were both voiced by Phillip Reich.
