What I find helpful is knowing the broad strokes of what I want to present/create. What the overarching themes are, if there are some tropes I want to lean on, how I want it to feel. Having a feel for this on various levels (such as personal, local, cosmic etc) puts me in a focused mindset. I have the broad picture.

Details then. I try to keep most within the larger picture but also make sure there are a few things outside it to spice it up. And here I find creativity to be one of the best tools, just making things up. Random tables can be really useful, even if you don't roll on them. For example I make use of Ironsworn's (pdf is free) Character Goal table pretty much all the time. Sometimes the goal is defining of them, other times it just sits there being an annoyance to them. Can also recommend the tables from Worlds Without Numbers (basic pdf free).

Then you will need something to keep your work organized. I use kanka.io as I found it having less bloat than some other services such as worldanvil. Browse around and try things out. Papers in folders also work for many.

I seldom find myself In situations where I have no idea what is in the direction the party suddenly goes into. The broad picture is there for me to rely upon. But I know what you are getting at. When inspiration fails I find there are two great methods. First being random tables, roll a bit and see what the fates have for you. Second is to draw on your players for inspiration, ask them questions and use the answers (a gm "rule" from Apocalypse World, read it when you have the time). Ask them what they expect to find over there, what draws their characters there. Do they expect treasure? Adventure? Try and get their intention for going there. Often what you get is the broad outline for a new adventure and you just have to realize it with them.

Good luck and do ask follow ups if you have them.