@Background Pony #BB5A: I’m well aware that not everyone would have read the comics. But Rarity!NMM has been pretty popular since her debut. And I personally doubt that it would be a retcon if NMM made a return in Rarity’s body once again on the show. Though “separate”, both the show staff and the comic staff have pretty much said that the comics and the show share a continuity. (Same with Equestria Girls.) So it wouldn’t make sense for events explicitly taking place AFTER Luna Eclipsed and Secret of my Excess, and between Magic Duel and Magical Mystery Cure, to be retconned into events in a later season. And since I’m sure that the showrunners would likely be aware of Rarity!NMM’s popularity, a sequel, with a flashback for those who haven’t read the comics where Rarity is possessed by NMM, would be just as likely if they decide to show her on the show. That’s my 2 cents on the matter.

@Pepsi_Al  
The major problem in treating NR as a sequel in the series, though, would be that not everybody has read the comics or even has easy access to them (I personally haven’t read any of them outside snippets). Particularly the target demographic is not likely to have trawled through the whole catalogue of IDW comics to know what the heck Nightmare Rarity even is about. And if you can’t have all of your audience on the same map without them needing to go through the extra expense/trouble to find out, then it wouldn’t be a very smart thing to do.
 
I personally feel the same about Equestria Girls. Let it exist as a spinoff, but I don’t care about it, so I have no interest in seeing the movies. And as such, I’d rather want to never see it crossing into the main series like some do since - from a completely legal standpoint - I’d then have to buy the DVDs in order to see something I might not even want to. Again, if you don’t have all of your audience on board and having them acquainted with the other thing, then mixing them together would be a very poor choice to make.

@FilthyRichFan25: Then those among Luna’s “fans” that prefer to see NMM as another side of Luna rather than a separate entity would all collectively scream out “RUINED FOREVER!!!1111!”.
 
@Lord Of Dorkness: More likely, they’ll build on it, and treat said appearance as a sequel to that arc rather than a “re-imagining”. Especially since the arc itself takes place sometime between Magic Duel and Magic Mystery Cure.

Every stallion gets nightmares about being forced to wear pretty dresses when she is in charge

@FilthyRichFan25
 
If she does, I’d imagine it as an, uh, re-imagining of the comic plot arc. Given how the comic is ‘B-canon’ to the shows ‘A-canon,’ so to speak, I’d near bet they want a clean slate for their take on the subject.

I’m curious if nightmare rarity would appear in any episode of MLP?