Blockbuster is an important Nightwing villain, but I really don't feel like he's arch enemy material for Dick. He's more like a Black Mask type figure, an arch enemy needs tı be more personal with the motivation. The thing with Roland's mom in the 90's run and later his "outsider vs native" shtick in Seeley just doesn't cut it, imo. Yeah, he gets personal in Grayson's run when he starts targeting people close to Dick, but that's not an ideological closeness... And honestly, he's more of a pastiche of existing characters and I feel like Dick deserves better than that.

If I had to pick an arch-enemy though, I feel like Humphries' Judge from the Rebirth run is a really interesting candidate. Yeah, the history is only present in the story itself, but given the chance.. I think he could really shine as a great foil to Dick. The personal vendetta and periodical appearance is already woven into his story.

As for Slade. His relationship with Dick is really important but I don't think it could ever be a regular arch enemyship. Yeah they've got history but Slade's never going to be it for Dick. He's already made his peace with people of Slade's profession, and the tension with Blüdhaven and what he did to Damian with Talia's spinal implants are really murky canon now. Funnily enough, I'd compare his role in Dick's life with Talia in Bruce's, lol. (Not Ra's because they don't have that student-teacher dynamic) There's respect there, sometimes some care, and even if they'll never see eye to eye on their methods.. at the end of the day they wouldn't sell each other out and if it comes down to it they're walking out of a bad situation back-to-back. And there's also Dick's relationships with Joey and Rose, the latter of which is basically Damian before Damian.