Whatever gets it out. Everything starts somewhere. Most of my stories start with a short story. Like five hundred to a thousand words of a scene. Then take that tone and themes and expand on them.
I haven't done a lot with branching paths, yet. The way I've been going about it is to not make choices. I don't make the choice, it's up to the payer. So, then I write both paths or I make a variable to show the choice and I reference it later.
What is most important is theme. If you are able to keep the theme in mind, even with vastly different choices, the story arch will feel complete and true.
For example the theme of the Great Gatsby is hope. Gatsby is a heroic hopefull figure. He hopes for an idealised past and projects it into the furture. It can't just that Daisy loves him-it has to be that Daisy always only loved him (otherwise it's ntr). Gatsby can't see things that don't fit into his hope for the future. Like, how Daisy and Toms life fit together (because they are horrible people), how Daisy will only want to be rich and Gatsby made his money off of prohibition contraband (Then again, it's easy for people with money to make more money), or the existence of Daisy's kid (the most heroically overlooked detail).
If there is a different path for Gatsby, the themes still should come through for a good ending. Like, if Tom dies before their confritaion then Gatsby would demand that she not greive--even though that will cut her off from inheritting her husband's wealth. So, she grieves publicly and gets her bag. So, Gatsby is left on his dock staring out at that green light, again.
Or, even a happy ending. Fortunatly, most avns aren't tragidies.
