It is also worth noting Miller is the first openly LGBT actor to play a superhero.
You're forgetting Alan Cumming, who played Nightcrawler in
X2: X-Men United. And Anna Paquin, who came out as bisexual in 2010 and then reprised the role of Rogue in
Days of Future Past (in scenes that were cut from the theatrical release but are being restored for DVD). And Kristanna Loken, who played the title role in the
Painkiller Jane series. And Victor Garber, who'll be playing Dr. Martin Stein (half of Firestorm) on
The Flash pretty soon.
A borderline case would be David Hyde Pierce, who dubbed the voice of Abe Sapien in the first
Hellboy movie (although that was before he came out). And you could make a case for David Yost, the Blue Ranger on
Mighty Morphin Power Rangers; I don't think he was publicly out at the time, but he was driven from the show by the homophobic behavior of members of the production, so he must've been somewhat open.
In animation, Neil Patrick Harris played Spider-Man in the 2003 MTV series, the Flash in the DVD movie
Justice League: The New Frontier, and Nightwing in
Batman: Under the Red Hood. Sean Maher also played Nightwing in
Son of Batman. Matt Bomer played Superman in
Superman: Unbound. And I'm not sure if he was out at the time, but Cam Clarke played Leonardo in the original
Ninja Turtles cartoon and He-Man in the 2002 series.
Cyborg will also be the first time an actor of color has played a superhero in a leading role since 2004's Blade: Trinity - although Michael B. Jordan will be co-starring in the Fantastic Four reboot slated for next year. It's the kind of diversity we haven't really seen in most Marvel movies recently.
Again, doesn't Momoa count? I think the Aquaman film is coming before the Cyborg film.