Tips for motivating myself when I don't know how to progress with a piece?

I'm working on a piece and I aim to be the first person to post a work related to a certain character in my fandom. But I'm writing all this based on a chapter of the game that isn't released yet so I've been struggling to come up with stuff to fill in the gaps. I know I could hypothetically just wait for the chapter to release in January, but I also don't wanna lose my steam for writing this by that time. Any tips for how to motivate myself to keep writing?