I also like the idea that superheroes are not going to be "illegal" in this world. They can't just do whatever they want without repercussions, but they are given quite a bit of leeway. I also don't think they are super rare, but you still don't often run into them during your everyday life.
Salt:
The proposed setup sounds good to me. What kind of power level are we aiming for with characters? What kind of ratio of adventure to socializing will there be for gameplay, and how much will be autonomous vs. GM-driven? Is there an overall theme in terms of flavor/aesthetics, like true to life, Marvel movies, comics, anime/manga, etc.?
Most of the answers to your questions will depend on what interests you guys. For power level, I would prefer less powerful than Superman (mostly because it might be tough to come up with good challenges), but other than that I am pretty flexible. As the GM I plan on presenting opportunities and challenges to your team, but it will be up to you guys how you want to react to them. Do you rush into the villain's lair, or scout ahead first, or nuke it from orbit? The choice will be yours.
For the overall theme, I am thinking something similar to the avengers movies where it is basically regular Earth overall, but the heroes and villains are a mish mash. Old gods, mutants, tech suits, aliens, and sorcerers all working together/against each other.
Again though, if you guys have alternative preferences, please let me know. These are just my initial gut thoughts on how I envision this game.
Here are some questions for you guys that can help flesh out the world:
What types of abilities do you want your character to have? Both fantastical and mundane. What might they have access too? (For example: money, tech, spellbooks, important contacts, or acces to a unique area)
What do you want your team to be called? Are you newbies or veteran supes? Are you a dysfunctional group or a well-oiled machine? Do you patrol a city, region, country, or the entire planet?