I think at the crux of any story is "change." Whether your character changes for the better, changes for the worse, or decides against change, that is the driving force. Change can be something inward, like personal growth, or perhaps something physical, like an accruement of wealth.

I recommend examining what your character wants, and if they do not have it, what will they do to get it? From that point onward, what will challenge them from getting what they want? Could be an outside force, another character, their own inner demons, etc. Personally, my advice is to try to find something that won't heavily depend on another player's character- but can be supplemented by another player's character, so that you don't feel stuck should that player's character not fit, go inactive, etc.