omg, this would be EPIC. a whole team of heroes gone horribly wrong. Except Pymtron got blowed up by Galactus. and Maestro would need to be stitched back together. but even Just Maker and Stevil teaming up would be scary as all hell.
As for Nightmare, we are assuming that based on the one tweet with the word Nightmare capitalized, it may have just been a typo or autocorrect (Marvel editor having nightmare capitalized by default would not be that unusual) and the other side of things is the 'Acts of' seems to be in reference to Acts of Vengeance, which implies a villain teamup, most likely with the specific villains appearing based on who they are going after. Could be one or the other, or Nightmare be part of the teamup.
I think the villain teamup seems most likely because the image they released is CLEARLY referencing the original Acts of Vengeance, with the exact same font and everything:
So, if Nightmare is a part of it, he'd probably be there because he'd be considered part of one of the targeted characters rogues gallery. It may point to Strange being part of things, in that case. but when I think Strange arch nemesis, I have thoughts of Mordo, or Dormammu, Nightmare is part of his rogues gallery, yeah, but... And i have a hunch the earlier suggestion that this is an Avengers corner event, or maybe even just an Avengers arc, is probably correct, so may have to scratch Strange. It does seem kinda early for a new universe wide event, and yeah, it's a good point that the X-Men are likely sitting this one out. (Though, rolling in the FF would be cool so Maker and/or Doom could make the cut) So Nightmare may actually be there cus Loki. Loki doesn't exactly have a sprawling rogues gallery cus he's new to the whole hero thing, about all he's got is Mephisto, Nightmare, (mostly due to what happened with Kid Loki, but still) Malekith and Laufey. (and himself) and assuming Mephisto remains imprisoned in Vegas, 3 of those are probably gonna be out for the count as of the War of the Realms conclusion. But think about it. Loki orchestrated the original Acts of Vengeance, what if the real twist this time is he has to (with the Avengers, and his brother) fight someone using one of his old plans (one that failed, because expecting Magneto and Red Skull to work together was never going to work, but still) against him? We know he's not gonna stay dead, and he's not turning heel again for the immediate future, his solo announcement spoiled that. It called him a hero, he's got Mjolnir, and it said Thor is helping him with his 'new responsibilities'. one of those alone could be explained away as a trick or something, all 3 together, but especially Mjolnir? he's pulling a full face turn. So. If he joins the Avengers in the rotating slot, assuming Blade leaves to do his own thing after War of the Realms, it would work. And we could at the very least have them facing Stevil too, cus Cap. The problem there is that a lot of the other characters on the team suffer the same skimpy rogues gallery problem, so it would potentially be a bit light on big bads, though it could lead to some lesser known baddies stepping up their game and
becoming big bads. Rolling the FF in could help solve that, too, with Maker and/or Doom.