I think it's a great idea to have some kind of general idea where you want the RP to go, regardless if the RP is sandbox, linear, or w/e. Obviously, it isn't as important in sandbox, but it's nice to have some sort of idea for those who would rather twiddle their thumbs and lump everything on the GM despite it being a sandbox RP. I'm not saying to literally railroad your players, as that would defeat the purpose of a sandbox RP, but that way you can give them a slight nudge rather than hear them go "But you didn't give me anything to write about!" (which has happened to me before).

Sections/acts/arcs are also a great thing to plan out as they give you an entire road map as to where you want it to go, but truthfully they're also useless because the site (and yes, I'm including myself here) is prone to falling out and/or ghosting RPs. You're putting in too much effort for something that'll fall apart usually in the 1st or 2nd page.