Critique and Advice; the treadmill of adventuring.

The easiest place to start is to look at what you already have. Maybe you don't need to come up with new ideas - you might be able to rearrange the ideas you already have.

For instance, maybe you've got someone with a clear motivation who's mostly a background character. What if you made that character the protagonist, either by telling the story from their perspective, or putting them into the protagonist's place in the story? Or writing that character out and giving the protagonist some of their traits.

Or, if there's some particular event that you plan to happen... well, you could make it the protagonist's goal to cause that event to happen. Even if this isn't revealed in the adventure until the event actually happens, it can work as a guidepost for you when writing it.

Alternatively, just pick something, anything, as a motivation and then ask yourself if it works. If it doesn't, ask yourself why it doesn't work, and then maybe you'll have a better idea what you're looking for.