IMHO, just remove the armor/terror charge mechanic. Even without that he'd still have huge damage, great range, and solid pressure.
Armor is always dumb, but it's especially dumb in a game that only bothers to give it to like, 3 characters. Very awkward and out of place.
No, he thrives on bullshit. He didn't have terror charge in SC2, but he was still intimidating.
What makes him fun is the ability to let your brain take a nap.
Nightmare is like the least intimidating character because when I see him I just think "Of course it's yet another Nightmare who's gonna do the same derpy shit." Compare this to Astaroth who is played about 0.01% of the amount Nightmare is played, so when I see him, I think: "Chances are this guy has put some time into learning how to nuke me in two hits." That's intimidating.
Again, I love Nightmare. That's why turning him into a cuck pick is so damn infuriating. He should be the highlight of a SC game, not some embarrassing, invasive tumor.
He's definitely one of the more difficult characters to deal with but I still think its too early to say he's cheap and needs nerfs. Then again, I'm a Mitsu scrub lol so what do I know.
That's the thing: It's not necessarily a matter of being "broken" or "needing nerfs." It's just about dumb design.
Think about Yoda from SCIV. Yoda wasn't a top tier character who "needed" nerfs. But everyone fucking hated him and wanted him gone. Why? Because his fundamental design was ridiculously stupid. He ignored many fundamentals and made you play his own stupid mini-game and it was wretched.
Dang, just went to the 8WayRun Nightmare Soul Arena and the first thread is: "So... can I have tips on beating Nightmare?". Player has the right idea to seek assistance as opposed to just bitch and moan about him. But yeah, Nightmare is derpy in SCVI. I mained him in SCV so I'm trying to play him honestly when possible. He can still feel a bit unsafe & risky occasionally.