This person may be a friend, an ex with whom you've always thought you may eventually get back together, or both or neither. You may or may not believe or acknowledge that you're currently interested in the someday girl, but when you imagine your future you can see yourself with her.
"Did loverboy tell you he likes Jules?"
"What? No. I just said SOMEDAY I could end up with a girl LIKE her."
"Jules is your someday girl?"
-- Cougar Town