Being a writer for DC would definitely be a fanboy dream-come-true. As for what I would write, well obviously there’s Shazam (shocking, I know), but since I’ve talked about that character–a lot–elsewhere, here’s what I would do with some of my other favorites:
The Legion of Superheroes: Restore the Retroboot team/continuity. The Legion Subs have finally been granted full Legionnaire status, albeit probationary. To make sure they stay in line and not cause any unnecessary trouble, they’ve been assigned a new field leader: a disgruntled Brainiac 5.
Red Lanterns: Starring Guy Gardner! Because this was a great premise that was done away with too soon, imo.
Hawkman, Earth-One: So I have an idea for a Hawkman story…it’s just one that would add yet another origin retcon onto the pile. So, instead, I would make sure to tell said Hawkman story in its own, separate continuity, just to avoid that mess.
Earth-3: The ongoing struggles of the Justice Underground as they seek to overthrow the oppressive regime of the Crime Syndicate, and bring peace to a world hellbent on destruction.
Question, Inc: Starring both Vic Sage and Renee Montoya, with appearances by Batwoman, Lois Lane, the Specter and Huntress. It’s the “X-Files” but with DC lore.