I'm not sure if this could be considered a spin-off, but I would love to see more about what each of the Winx's lives were like before Alfea. We've had some flashbacks to their childhood, but I would love to see full prequels. Short, but complete films about each fairy’s childhood and what shaped them into who we first saw them as in Season 1. 
 
 The same goes for the specialists, btw. They gradually became quite important characters in the series, yet we barely know anything about them, that's so unfair!  

 I would love to see a brief backstory of their lives too (especially Riven honestly  

 )