Haley was seemingly set up as an ideal character for a spinoff, although the Modern Family co-showrunner has stated that a spinoff would probably revolve around multiple characters.
Of course, if a Modern Family spinoff centered on Haley does eventually happen, Sarah Hyland may be called upon to go back to her brunette look.
Then again, Modern Family itself didn’t end on a flash-forward, so a potential spinoff could do pretty much anything with Haley without contradicting anything in the finale.