How can you design a user flow when you don’t know the functionality?

I am currently designing a user flow diagram on lucid chart. I am working on the "search" section. We know we want a normal search e.g. user types in their keyword or keyphrase, but we also know we want an advanced search too where user may filter by how they want to "feel" e.g. happy, excited etc.

This is not certain, we may have multiple, or singular ideas for this - how do I design without knowing this info?