we are starting out medias res so the Earth Green Lanterns have been around.
Green Lantern has a okay bench of stories to pull from without cameos and stealing other characters. But also the space characters like Adam Strange who probably never will get a series or movie could organically appear.
The space cop angle is pretty flexible in terms of introducing characters I think.
Yea, i think what works with the Penguin is that the framework honors the lore but the story and take itself is pretty original, with the perfect tone.
So with GL we know the Corps can/has been analogous to a crooked police force, but also to military stationed in places where they are unwelcomed. It's had horror elements like Aliens, dire horde battles like Star Ship Troopers, swashbuckling akin to Star Wars (bar of drunken alien mercs/bounty hunters), and even some detective murder/mystery stuff too.
So i agree there's a lot of flexibility with the IP in general, it'll be about narrowing in on the best route and tone for this John/Hal teamup.