I don't tend to write stories with what I called 'blockbuster' plots; you'll find little adventure, saving the world or action scenes in my stuff, I'm no good at it. A story in which most of the major villains get together for some world-ending plot like the MLP Sinister Six is pretty far outside my wheelhouse. But I've had an idea for a scene in that kind of story knocking around in my head for ages.
It would be one of the early scenes, in which several villains (probably a 'Greatest Hits' lineup: Nightmare Moon, Discord, Sombra, Chrysalis, Tirek, you know the ones) convene and start forming the idea of all working together. But supposing some of them didn't trust Nightmare Moon because she'd so recently been one of Equestria's protectors? At most, Discord would get ribbed for having a pony marefriend and being a side-switcher, but everyone knows he's fundamentally untrustworthy all around, that's just who he is. But NMM used to be one of the princesses who worked to imprison at least three of those others. Sombra and Tirek especially wouldn't want anything to do with her; even Discord, as jovial as he is, might have a bit of anger still about being sealed in stone. Nightmare would certainly be insulted and yell about how that was all Luna, not her, and she can't be blamed for any of that. It might still cause some friction in the group after they move past that point.
It's not a whole lot, I just really liked the idea of that: after Luna faced so much distrust and fear because she used to be Nightmare Moon, wouldn't it be interesting to see Nightmare Moon called into question and suspected of treachery because she used to be Luna?

Yeah. Have to admit, this DOES look like it could be really intriguing. Especially since you wrote this idea about three years BEFORE the actual final season, which DID involve a team-up of most of the major non-reformed villains.