I feel like I've answered this one multiple times, in increasingly obtuse ways.
When I have an idea for a whole fic I just write the very basics of the scenes in notepad, so I have some idea of what needs to happen to make it flow. But I also make random assorted notes in other notes apps if I have ideas for dialogue or whatever and I'm not at my computer.
I only have 5 WIPs right now, but I do have basic plotbunny files for a few other random fics. Mostly crossovers that I'll never get around to writing - there's a couple of Worm crossover ideas that are just "these are other franchises I've never seen crossovers for despite obvious plot implications" and nothing else.

Reposting because Retrospring just renumbers lists to be from 1 to 5 so I can't do countdowns. Hopefully this works. Some are just arbitrary picks because I can't think of many who I like more.

5) Professor Sada from Pokemon. We know next to nothing about her at this point beyond buff cavewoman(?) professor, but it's hard to make a list like this without mentioning her right now. Bonus points if she turns out to be the villain of the opposite version because then I get TWO characterisations of her.

4) Mudrock from Arknights. I just respect her vibe a lot.

3) Alice "Daisy" Tonner from The Magnus Archives. First, ACAB. Second, this is a podcast, so I have to take the word of narrators and fanartists. But third, I am gay.

2) Barghest from Fate GO. Tall buff lady with sensible (for her proportions) platemail armour. Has to hold herself back from eating those she loves. Wife city.

1) Tifa from FF7. The classic. If you don't play FF7R with the Buff Tifa mod like she deserves then you're not playing it properly at all.

Buddy, if I knew I'd tell you. My brain has been keeping me up until 3am and making me write entirely new fics in a different fandom from the longfic I'm so close to having 5 new chapters for. I just need to do some connecting scenes I can't focus on.
Some people find it helpful to make outlines of every scene they want in a fic so they have the framework to jump around in making it as inspiration strikes, some people have fic-writing playlists that may even be specific to the vibe they're trying to capture. Meanwhile I have my brain singing showtunes to me several days in a row and offering me outlines for fics in completely different genres.
The only thing that consistently works for me is making notes in a reminder app when I have scene ideas, and folding them all in later. This is apparently completely bizarre to most of my friends.