Reach deep into your fanboy/fangirl soul and find the one plot point you most want to change. From Star Trek to the Brady Bunch, from midichlorines to Andrew Keaton.
For me I would erase the death of David Marcus in Star Trek III
I liked him. I connected him. And before Enterprise, before Voyager, even before Deep Space 9 I had envisioned a series around a young James T. Kirk struggling to deal with the fact that he’s named after one of the Federations most famous captains. Struggling with all the expectations that would come with carrying that name. Which is an idea that would work best if said James T. Kirk was the grandson of the original, perhaps named so after the Enterprise-B incident? Surnames being a little more freely used in the future. Poor David.