@FelixMurikiCodeman
Q1) If you want to create an ensemble cast of characters, you can just pick some other theme - e.g. Greek goddesses, animals, planets, meteors (you can make up some characteristics of each fallen space rock)... weather and elements has been done but perhaps pick some other elements like gold, platinum, uranium... Or maybe they don't all fall into a theme. Think about the Spice girls, while not superheroes, they each had their own identity, i.e. Sporty, Baby, Scary, Posh...
Q2) Kevin Smith once said that comic book stories are mostly second acts. Meaning, when you buy a random Batman comic - it assumes you know Batman's origin, and Batman doesn't die at the end. The story being told is all second act. So perhaps you could assume your audience knows something about your characters and you can write a series of short stories that have a beginning, middle, and end, but aren't these big epic adventures. Think more like an episode of TV rather than a series or a movie. You can write a short 10,000 word story that has some arch, but mini arches or subplots with each character. For instance, one character's story arch could be that she lost her favourite sweater, another got a bad grade on an exam, another is feeling ill, and another is getting to know a new neighbour. These are the kind of low-stakes scenarios we often see in a typical episode of a cartoon - something that can wrap up in 20 mins. If you write enough of these, you could stitch them together in an anthology and people will know so much about your characters personalities that they will fall in love with them and want to see them in other scenarios. I think there should also be a larger plot of them taking down some evil doer - after all, they are superheroes. I think a lot of people get stuck with finishing because they don't really have a clear direction on where they are going. I like to plan everything out in a detailed outline before I start writing.