After extensive play with him this weekend, I can tell that Nightmare is top of the mid tier.
He's all about gimmicks and how good your opponent's anti-NM is.
All weekend, I was getting free KOs and RO's from half way across the ring because of his new doom "combo". That's GSK BE, GSK BE, GSK BE, GS KE BE. Because of the new ukemi system, many people will try to ukemi by holding the direction they want to tech and G, but instead they will tech late or begin rolling, which guarantees a GSK BE or 1B. Its amazing. The same mix up can be applied to WR A.
IF they do start teching, 1A will catch sometimes, depending on the distance, but will always lose if they tech to Nightmare's right.
Also, 1A after 33B BE will not catch Nightmare's right, but if you step to Nightmare's right and instantly 1A, it will catch all tech and grounded. If you input 1A too soon, you'll get 11A, but it will still catch all tech, but won't hit grounded.
2B+K is GODLIKE. Its much harder to react to than what it seems. The tremer is also a GB, so your at advantage no matter how they block. If they start jumping, the mid has such a weird hitbox they might get hit jumping towards NM, giving you an incredible groundstun. This move is also a tech trap and/or force block in soooo many situations, namely after 2A+B. If they roll after 2A+B and stand up, they'll be right at distance to get stunned by the tremor, but if they stand immediately, the won't be. Even if your opponent knows its a special mid, getting up into this move is very, very scary.
33B, A+B can be blocked if they fall behind NM apparently, which sucks. Now he's got no go-to answer post-33B. You're best bet is to either learn to eyeball the range from which you hit 33B or press 2 or 6 to turn/step around them and get what you can. You could also just cut your losses and stick with the GS B combo for 65ish damage.
1A is also one of his best moves for oki now. I haen't tested it thoroughly, but because of the new commitment to rolling, it seems that 1A is guaranteed after many knockdowns if your opponent decides to roll.
I LOVE 44A. Its a great whiff punisher and rush-in killer. After it hits, GSK BE picks up, 1A is a tech trap and 33B is guaranteed oki.
MammothMan discovered something godlike: if they do anything but immediately guard or tech, his CE is guaranteed after 33B BE. Because of this, if they stop teching for the 2 ~ 1A trap, then they'll eat a CE for 200+damage.
And finally, 4A+Bg is much more useful now. It literally disguises half of his most useful moves. It hides the animation going into 2G, which gives access to WR A and FC 3B. It also blends into 2B+K BE for sooooo many WTF moments. and after so many UB cancels, your opponent will eat a 4A+B or 2[K] which is much faster now.
I love this Nightmare so fucking much!