I has suspicion idea that who may kidnapt Flurry Heart might be a another revamp villains which we saw from season 5.
First is the Smooze

Which he eat is jewels funny that race that can defeat him in g1 has jewel called the Sunstone make their land alive but if removed that their land would turn into this.

So got an idea that going to a revamp virson of the witches of gloom.

5031559
I like the idea!

5031559 I dunno...she's the heir to the Crystal Empire, I mean who has the most motive to kidnap her as leverage to take back the throne?
Uh well, yeah, geeze, nobody we'd know...

5070557

5070573 True evil never dies! hehe! That and his horn is still a thing...

5070584

He already returned. Conquered the Empire again, almost doomed Equestria to the apocalypse, then chose the good side in the last moment.
He's not Equestria's enemy anymore.

5138992 Yeah, that's what I thought when I read the comics (which I loved both the Fiendship is Magic and Reflections arcs), but the end of Season 5 (Cutie Remark) still regard him as an evil pony. It's great having a character you can write for that you can either continue writing great evil done by his hoof, or write him as redeemed. Seriously, how cool is that???

5139166

The events that happened after Starlight Glimmer manipulated Starwirl's spell sure provided opportunity to show an evil King Sombra again.
And yes, the other King Sombra in the mirror universe has become evil and still is probably. But it would be hard to make episodes about what is happening, most likely even impossible, because Twilight and her friends can't go there anymore and the other side has neither a way to reach them.