I like the idea of a later setting, like the eighties or nineties (maybe late nineties). Also, how common are supderhumans (or whatever other species you might want to be) and are they organized?

In response to those two questions I suggest it being uncommon for people with powers to exist. As interesting as a world filled with super powerful beings might be, I think a quote from The Incredibles sums it up well enough. "Everyone can be super. And when everyone is super, no one will be."

As for them being organized or not, there are positives and negatives to both of those options, and they both bring different dynamics to the table depending on how they're played. On the one hand we have groups like the Justice League, Avengers, and (though I'm not too familiar with them, I believe they fit the bill) Watchmen. On the other we have a series of separated/unaffiliated individuals/groups of superheroes presiding over their own "territory". Or we could do a combination of the two by having separate larger groups protecting larger areas or countries that are further broken down into smaller areas and protectedby individual heroes or teams.