Agreed! This would be fantastic, as they already have a good basis for superheroes/villains from the Batuu pack, Sims 1 and 2's Captain Hero rank in Law Enforcement (in Sims 2, they'd fly away to work like a superhero), and Sims 3's Movie Stuff pack. The world could have at least 2 64x64 lots for extravagant and vast mansions (similar to the X-Men's mansion) for superheroes or rundown grungy locations for supervillains to hide in (though this could be reversed, is up to the player):
Perhaps there would be at least 25 skills in the shared Supersim skill tree, with multiple ways to become a Supersim: from accidents with the chemistry table, to drinking too many potions of Curse Cleansing or Good Fortune in a short period of time (probably 5 in a 2 hour window), or maybe reaching the top of certain careers and their branches (like the top of the Criminal Career in the Oracle Branch, or the Space Ranger branch in the Astronaut Career), along with a bloodline trait like spellcasters that allows for it to be passed down to future generations. Kids should also get full access to the powers, as long as they wouldn't be inappropriate for the child to have (like laser eyes awa), and access to the skill tree as well. Perhaps a few traits could be added in as well, like Fanatic, a sim who is obsessed with the glitz and glamour of Superheroes/Superpowered sims (like werewolves, vampires, ect) and can "Fansim" over them, acting similar to the celebrity system from Get Famous, but with unique interactions, like "Ask to Fly Sim to...", "Ask to be Flown Around...", "Ask to Show off Powers...", Ect; gaining special moodlets depending on if the receiving sim likes Fanatics or dislikes them, like "Antsy about Flight", "Woah! Birds!", "Nearly Dropped (insert pronoun tag here for code idk it yet)...", ect. There could also be sims who fight against supersims, like a protest system similar to City Living, where sims could learn to be Wary: Similar to Paranoid, but more focused on focused and tense moodlets of beings different to them, like Spellcasters or Ghosts, being able to be influenced to like or dislike certain lifestates as well.
This is just off the top of my head though!