A Martian Manhunter film could be fun against a 1950s backdrop of the red scare/McCarthyism paranoia. Include King Faraday as a FBI spook.
A 70s Green Arrow story could be pretty sweet too. It was time of social revolution and protests in US. I’d love to see Ollie picketing the man (Nixon, Watergate, Vietnam— take your pick!).
Luthor in the 80s, in the same vein as Oliver Stone’s Wall Street (with either investigative journalist Jimmy Olsen as our POV/Charlie Sheen character, or someone else).
A Booster Gold film with him and Skeet getting stuck in the 90s sounds like a fun time too, especially if they’re paling around with Blue Beetle (outfitted in Radio Shack/90s-ified tech).
And lastly, it’d be great if DC could figure out how to make an Icon film. Have the film toggle between 1800s southern US and the present day from the perspective of Augustus Freeman/Icon.