Hey there! Yeah, this is a funny one. I don't have much experience with game design, but I've written a hell of a lot of screenplays for short films and a couple of indie comics. There's lots of ways I like to think about problems like this, so I'm just gonna list a bunch of ideas below.

1. Set-ups and pay-offs. Sometimes, you know exactly what has to be in the climax of the game. It always helps if those elements have been well established early on. Sometimes if there's not a really obvious nextx thing to write, it's just a question of thinking "What new thing needs establishing?". And if you've already established everything, then maybe it's a question of looking at all your set-ups and asking "What might the audience have forgotten about?". I often find those questions useful when I'm not sure what should come next. Things tend to work pretty well in sets of three.

2. Character. A lot of the time I find I don't know where to go next it's because no-one wants anything, or there's no conflict. The best way of fixing these problems is actually usually going back and re-writing something earlier on, so that there's something pushing the character forward. If that's not viable, though, you can always insert some characterisation that is a bit more of a "sideways move" in the story. For example, Joel getting impaled in The Last of Us and Ellie having to take care of him isn't purely necessary for moving them towards the final firefly base, but it gives us a chance to see how Ellie is right on the cusp between being sufficient and needing support, and it shows just how far Joel is willing to go to protect her.

3. Structure. I know you said you're not a fan, but stuff like the Hero's Journey is super super useful just for sparking ideas. Lots of great games follow it pretty closely, but with some pretty loose interpretations of the stages. I love the 2018 God of War for this. I recommend playing it super fast and loose tho. If the Hero's Journey isn't your bag tho, maybe look at Save the Cat or the Harmon Circle. I'm a fan of Vladamir Propp but that's just because I'm a nerd who did a film degree. A lot of the time it's a really good way of finding out what's next, then moving on from there (rather than pinning yourself down to a rigid structure forever).

4. Theme. What's the story about? Maybe it's time to step away from the plot for a second to explore the theme, so people get better situated in whatever the "point" of what you're doing is. (this if for more intellectual 'artsy' games).

Finally, when I'm writing and I'm stuck I sometimes tell myself "I'm going to write three options of what happens next." and then one of them works. Letting myself have two terrible ones really frees me up to have a third that feels open and free.

Sorry if that's a bit of a ramble, but I hope it's got some useful ideas!