One idea could be an OC with a unique Stand ability that is related to time manipulation, but in a different way than Dio's or Giorno's. For example, the OC's Stand can create time loops within a small area, trapping enemies. Another idea is an OC who is a descendant of a minor character from the original series and has a vendetta against one of the main villains because of something that happened in the past. A third idea could be an OC with a Stand that can transform into different animals, each with their own special abilities, and they use this to navigate the strange and dangerous world of Jojo.
To create a memorable character for your Jojo OC fanfic, start with their backstory. If your OC has a tragic past, like losing their family to a Stand user at a young age, it will shape their motives and actions. Their physical traits can also be important. Consider giving them a physical quirk, such as one eye being a different color than the other. In terms of personality, make them complex. They could be kind-hearted but also have a short temper when it comes to injustice. And their Stand should have a unique mechanic. For instance, a Stand that can only be activated when the OC is in a state of extreme calmness.
One common theme is power combination. Since Jojo has such unique stand abilities, when crossed over with other universes, seeing how those stands interact with different powers like magic or superhuman abilities from other shows is always interesting.
Well, the 'jojo's bizarre adventure novel' has a really complex plot. It spans generations of the Joestar family. There are different arcs, like the one where Dio Brando is a major villain. He has his own evil plans which often put the Joestars in great danger. And the characters keep evolving and facing new challenges in different locations around the world.