In the last scene where Mia and her husband walk into Seb's, she finds out its the jazz club Sebastian had always talked about running (Which he wanted to call 'Chicken on a stick' previously). After Sebastian finds Mia, he starts playing the Piano and the whole scene that follows is what could have been Mia and his story together, which is quite heartbreaking. Are these thoughts only in Sebastian's mind or is even Mia thinking the same?
In my opinion, it's pure speculation whether all, or some of the scenes belong only to one of Mia's or Seb's imagination. The ending is bittersweet, Mia and Seb "live" their "could have been" lives in those moments.
For the viewer, it is irrelevant whether those scenes are imagined by one of the characters in the movie. He/She is nonetheless forced to think, to wonder, and to dream of what life "could have been" for them.