Writing is hard, especially when you have a lot of cool ideas. For me, the best way to get motivated is to write whatever cool idea you're thinking about the most! After that, start writing whatever details can help make the story more nuanced!
if you can't come up with anything, try using writing scenarios for your characters to flesh out their personalities and how they interact with each other. I personally like doing this because it helps me imagine my characters in the world I'm building.
Good luck writing! (*^▽^*)