Lots of good ideas here. Cheyenne Jackson would make a GREAT Frank and he did recently star in Pinkleton's La Cage. I'd pay to see it! There was an (unsubstantiated) rumor in past threads that someone at Roundabout wanted a Black Frank. And should that be the case, I think Sydney James Harcourt would be divine. Honestly there are SO many queer Broadway actors who could nail this, so as long as they don't go down the (imo far fetched and ridiculous rumor) route of Timothee Chalamet, I'll be happy.
Other ideal candidates:
Riff Raff: Adam Lambert
Narrator: Barry Bostwick (inspired idea and now I can't picture anyone else)
Magenta: Amber Gray
Columbia: McKenzie Kurtz
Rocky: John Riddle or Timothy Hughes (I know they aren't the usual look, aka not blonde, but let's get a talented sexy hairy dude in the classic gold speedo!)
Brad: Matt Doyle, or Conrad Ricamora, or Jonathan Groff
Janet: Andrea Macasaet, or actually McKenzie Kurtz could nail this role too