Ultimately I think that bringing Xavin back would throw a giant axe into the story that Rowell is trying to tell with Nico and Karolina too. Xavin was inarguably the love of Karolina's life, and then having them come back after Karolina finally got a chance with her first crush... that'd be even messier than 19 year old Chase being in love with sixteen year old Gert.