INteresting question. Funnily enough, sometimes the first thing I get is the setting. I’m very into the atmospheric setting & its role in the story, so that’s definitely there before I start any planning or research or even think about characters! The other thing though is a pretty formless idea of some sort of ‘issue’ I want to explore. Not big grand issues, but something little like (current wip) the conflict between your adult self, and the child-self your parents still see you as. Or like the story of St. Kilda (last wip).
Those two might happen in either order, but I need both before I can do anything else at all.
Half ideas are frustrating! Maybe, I don’t know, read some poetry dealing with linked emotions, google images, make up a character using one of those model things, and then see how they fit with your half idea? If none of those work, feed it tea and chocolate til it breaks down and confesses.