The world of Jojo's Bizarre Adventure has occasionally given the various Jojo protagonists a chance to interact: Joseph and Jotaro in part 3; Joseph, Jotaro, and Josuke in part 4; and Jolyne and Jotaro in part 6. One of the series' biggest missed opportunities, however, was their chance to bring Giorno into the loop in part 6, Stone Ocean.
Since Jojo is a family saga, characters are usually direct relatives in some way. Joseph is Josuke's father, Jotaro's grandfather, and Jolyne's great-grandfather, for example. The one major outlier is Giorno Giovanna, the protagonist of part 5, Golden Wind. This is because his connection to the Joestar family is a bit tenuous. When DIO took over the body of original Jojo, Jonathan, the body apparently remained perfectly fertile, and DIO's use of his vampiric abilities led to many a seduction throughout the 1980s. One of these trysts resulted in Giorno's birth, and the star-shaped birthmark confirmed that he was indeed a member of the Joestar family, despite the technicalities. While Jotaro at least was aware of his existence, Giorno doesn't interact with Jotaro directly at any point, nor any other Joestars.
Chapter 103 of Stone Ocean has the villain, Enrico Pucci, come into contact with a trio of young men. He identifies them as the "Children of DIO," which turns out to be more literal than expected when it's revealed that each of the three also bears the Joestar birthmark. Like Giorno, these boys were the result of brief, damaging affairs that DIO had over the years. Pucci tells them that they've been drawn here by fate, even beyond that which normally draws Stand users to one another, and convinces them to live up to their heritage by helping him to enact his plan. They set out to stop Jolyne and her friends, giving Pucci the time he needs to complete his scheme.
These three men are the only antagonists to have the Joestar birthmark, at least prior to Jojolion. As DIO's children, they're all half-brothers not only with each other, but with Giorno as well. If these three were "drawn" to Pucci, why wasn't Giorno also summoned to Florida at the same time? And while all three of them chose to go along with Pucci, they did so willingly; even if Giorno was drawn there, that wouldn't mean that he'd have to act as an antagonist to Jolyne. In fact, it would have been an excellent reason to have Jolyne and Giorno work together, especially since Giorno would recognize her father's name.
As cool as it would have been to see Giorno and Jolyne fighting side by side to save Jotaro, there's one obvious meta-fictional answer as to why this wasn't the case. Simply put, Giorno's Stand, Golden Wind Requiem, is simply too powerful. Had Giorno been brought into the story, he could have easily defeated Pucci and ground the story to a halt. On the other hand, Jotaro's Star Platinum is also insanely powerful, but that didn't stop him from being brought into the story. Jotaro is out of practice with his Stand, bringing him down to a more reasonable power level, and there's no reason that couldn't have been the case for Giorno as well - especially since Stone Ocean is set 10 years after Golden Wind. With the alternate continuity of parts 7 and 8 now the default setting, it seems especially unfortunate that Jojo's Bizarre Adventure never let Giorno meet the other Joestars.