“ She’s an Alicorn who plays a pivotal role in saving Equestria from King Sombra and his minions - “
This is very strange, it must be a mistake because we did not see a villain in the movie, only Sprout who was more of a wandering disaster on four hooves than a villain. It’s also a little too unoriginal to let Sombra return in the G5, he is a heartless tyrant with megalomaniacal dreams as the most archetypal evil pony in the Ponyverse. But he has exhausted his potential, if Hasbro was smart, they should have let IDW “clear him away” for good before withdrawing the license. Moreover, Sombra will be very strongly misplaced in the modern Equestria, he belonged in the medieval Equestria when Celestia and Luna were young.
Sunny and her friends do not need old and exhausted villains from the G4, they need new villains from their own era - and timeless villains like Grogar, who only appeared in the G4 once. Sombra is too low on the hierarchy among the villains that can appear in the G5. And it would be heartless to let the Terrible Trio appear in the G5, it meant that they have lost everyone by waking up in a world that is not theirs.
I find it very unlikely that Twilight, who in the IDW comics captured Sombra, will make him into other people’s problems as Celestia had done. She will probably reform him or petrify him and put the statue in Tartarus if it does not succeed.