I'm talking about things after you determine morality and how four-colour and gritty your campaign is, when you're doing the world-building.
These things are in no particular order. I've numbered them just so that I can easily refer to them.
- How common are supers? You have to have enough for your campaign, unless it's usually against Team Rocket....er, the Brotherhood of Evil Mutants. Do they concentrate in this area, like New York is apparently where 99% of Marvel supers go?
- Where's your home base? East Coast, West Coast, Midwest, Great Lakes, elsewhere in North America, not in North America? Existing city, existing supplement (such as San Angelo or Freedom City), or made up city?
- Who deals with the remaining evil lairs after the supervillains are cleared out? Is there a government organization, probably part of US Army or National Guard, that's dedicated to "sanitizing" abandoned evil lairs? Are there thrill-seekers (a la Base Raiders) who want to get there first, or portions of Dark Web that sell scavenged high scientist tech?
- It's always nice to have an undertown, like Seattle or Chicago or Stark City, whether it's a place for criminals, mutants, evil lairs, or just Bad Things.
- When did superheroes appear or go public? Around WWII? The Roswell crash? At Kent State? Did a superhero greet Apollo 11 on the moon? That'll help determine the existing changes to history, if any.
- How do superheroes deal with cellphones and built-in GPS devices? Is there a company that sells hardened cellphones for supers, with a discount if you post on Twitter-Facebook-Instagram-etc. (The company also sells location info of supers to government.)
- Are there super fabrics for costumes? How do supers get their costumes. Idea: there are super fabrics for costumes, with multiple manufacturers, though some skimp on it and there are knockoffs, so some costumes get torn easily. There are tailors/manufacturers for super suits, but that tends to fall into government/pricey or underworld manufacturers. There are non-hero reasons to buy the fabric, so you're not going to be pegged as a super just because you buy a few yards. (It's tough & fire-retardant, for starters, though it has terrible insulation properties. (Which is why you can be athletic as heck without overheating.))
- Does Atlantis/Thule/Latveria/Bialya exist, and does it have a seat on the UN council? Idea: Atlantis is trying to get a seat at the UN but countries won't recognize it because doing so would mean that Atlantis has a say on deep sea oil drilling.
- Do animals ever manifest powers? Is there a Legion of Super Pets, or do you have to be a hominid (ape or human) to get powers?
- Is there a "hero gadget underground" where profs assign hero-gadget tasks to (engineering) students and if there are heroes who take them on, a commensal relationship develops. Or a subculture of "supermakers" who are trying to duplicate or improve various pieces of hero or villain equipment.
- Traditionally, superheroes don't change the iconography much (people in our world still have to be able to read and enjoy the comics), but are there a couple of characters who are, like, revered (whether living or dead), like Superman or Captain America are in DC & Marvel? They don't have to be in the game themselves, just names as currency.
- Do heroes trademark their looks? Or is it automatic? How are they affected by changing costumes? Is there a law that trademarks superhero identities for them? Or is it like that clown archive, where each clown's makeup is painted on an egg--the identity can still be misused by others, but those in the know can see who owns what identity?
- Are supers common enough that there's a place to go if you suddenly discover you have superpowers? I'm not speaking of Shady Dan's Den of Powers, which will show up no matter what, but rather a real training thing? Or do you wait to be apprenticed by an existing superhero, or make your own way, or get kidnapped by the Men in Black? Or all of them?
- Have superheroes been around long enough that there are retirement homes for them? Who takes care of a guy who (say) has Alzheimer's but can still level the block? Who visits? What secret information gets spilled in such a place?