Sara's Father is DIO.
Sara's Father is Hol Horse.
Hol Horse has had girlfriends all over the world, it's possible he could have an illegitimate child he doesn't know about. An idea that's also been explored in the Blood Sun Vendetta series. There's enough of an age difference for it to make sense, she's the first of the cast to meet him which seems like set up for a reveal, their names are both references to Hall & Oates songs, her being Michelle's friend makes for a nice contrast when their Dads were rivals, finally Sara and Hol both have gun like stands. Sara's Father is not any canon character.
If Iron Touch takes place in 2009, and Sara is 21 years old, then she was born in 1988. This is helpful with eliminating possible characters, as the vast majority of characters in Jojos Bizarre Adventure Parts 4-6 are either too young to have conceived her or are not nearly relevant enough to make an impact as a reveal. Part 1 characters are all essentially dead by 1988, and part 2 characters are both very limited and very old, the only legitimate candidate being Joseph (unlikely due to Hermit Purple never revealing anything about Sara and her not having any known Star birthmark). Because of this, it makes the most sense for her father to be from Part 3. Jotaro and Kakyoin can be eliminated under being minors, Joseph has already been crossed out, Sara does not recognize Polnareff as her father when Michelle shows his photo to her (even implied to find him attractive based on Polnareff’s comical muscularity), and her appearance contrasts greatly with Muhammad Avdol. Iggy is a dog, so his status as Sara’s biological father is incredibly doubtful. The rest of part 3’s male characters are mostly one off villains with no weight. Dio has had many “brides”, but Sara has not mentioned any star birthmark that she would have if this was case, something she is very likely to do. The most common, and most likely theory is that Sara’s father is Hol Horse. However likely it may seem, Hol Horse’s purpose in the story does not seem to be directed at Sara. Rather, his most meaningful interactions seem to be with Michelle, seeing as he knew her father quite well. She is incredibly suspicious of this, an unresolved plot point that is very closely tied to the overall goal of finding out what happened to her father. Having Hol Horse be the recipient of two major plot points involving daughters seems repetitive, and it seems as if his arc will favor Michelle’s conflict rather than Sara’s.In the end, it seems as if Sara’s father is an original, noncanon character, who will be largely significant in some other manner. Boney's character will be based on The Joker
Word of God has stated that Boney is an Expy of a certain green-haired character that's not Beetlejuice. They're both green-haired major enemies seemingly without any powers. Bad To The Bone even gives Boney a form of Joker Immunity. One of the main cast will be turned into a Masquerader at one point.
Most likely Hol Horse Depeche's Soul is Tied to Policy of Truth.
A reason for spreading out the painting after his death might be that he has in some way tied himself to it, giving himself the best odds of surviving and influencing events even after his death. Cascada is a descendant of Captain Tennille from Part 3
An aquatic themed stand which travels through water, belonging to someone who has links with SEES, an organization which has huge amounts of links with DIO's followers. And judging by what bits of loyalty remain for DIO in Midler's eyes, her taking on someone as a family favour isn't too far out of left field. The Grand Marshal is a Vampire.
Little is known about the Grand Marshal's identity, but this theory has become prevalent within the fanbase after one of the latest chapters. In Chapter 30, Boney makes a remark about the Grand Marshal that implies they're a night owl. However, some have taken this line further to say that this is because of their vampiric nature. Additionally, Boney is very cautious of the Grand Marshal's safety. This could be because it's unsafe for them to come out during the day (much like DIO). The existence of vampires has been a running gag throughout the story, so to make one of the major villains a vampire would be quite the payoff. Bufala let out the Grotta Azzura Vampire.
With the recent release of "pierces.exe_11012006,"
we now know that a vampire escaped while being transported by the Speedwagon Foundation and Passione. This makes sense given how Chapter 21 revealed that Passione and the Speedwagon Foundation had strained relations after their latest collaboration. However, what if this wasn't an accident? What if Bufala, who we know once worked for Passione, actually set the vampire free himself? Could this have been an act of defiance against Giorno's leadership? Additionally, it's possible that the vampire was responsible for Bufala's missing eye, but that's pure speculation. The Akira Otoishi concert will lead to the Grand Marshall reveal.
Based on a few factors, such as Michelle's birthday and Word of God in the Author's Notes for Chapter 29, the Akira Otoishi concert in California two weeks from the recent chapters' timeline will lead to a 'turning point' in the story. It's possible it will not be the case, but there are signs that: not only will Michelle and the gang cross paths with Giorno in possibly a Stand-users-attract-Stand-users situation, but that there will be a significant Masquerader presence in the concert... especially if the Grand Marshall themself decides to appear. SEES let out the Grotta Azzura Vampire.
Based on revelations from Chapter 37, it seems like SEES would have incentive to try to steal the Speedwagon Foundation's vampire. Mr. Williams desire to find immortality for Lucky, his cat and user of Random Access Memories, could be motivation for him to send out SEES assassins to steal the vampire from the Speedwagon Foundation.