I have been wanting it a long time, and the sims series has flirted with it since the very first game with a super sim costume, sims 3 had a whole theme in a pack for it too.
They probably should not make their powers too complicated though, streamline it a bit. I'd use superman as a basis, like strength, durability and flight could be for all of them. They could have to learn to control their strength with a perk to not keep breaking stuff by accident around the house.
Super sims could have a list of minor powers to learn that all super sims could learn, but could also pick one major power which would be their thing and locks out all other major powers. Could be a branch with three steps, unlocking better powers of that branch as they go.
Fire powers, ice powers, mind powers, storm powers and transportation powers would probably cover enough styles for major powers.
With fire powers they can't just put stuff on fire but could also dismiss the flames too when there's a fire happening. They are immune to fire and can walk right through it. They can also breathe fire, juggle fire balls and otherwise impress and entertain other sims in a variety of interactions, with a small chance of putting something or someone on fire if they are not careful.
Ice powers could let them craft a whole furniture set made of ice, which won't melt. They can also freeze sims, which won't age while frozen. A sim who masters ice powers could no longer age. They can also cause the weather to be like winter where they are, any time of year. They can toggle it on and off.
Mind powers could let sims easily learn the traits of other sims and make them do stuff, they could however at times when they try peek into another sim's mind encounter stuff they did not want to know which is likely to put them in a specific emotional state for a while, explained by a humorous moodlet. They can speak telepathically to any sim they know too. They do not use a phone, it is replaced by a purple orb they hold which lets them do all the same actions telepathically.
Storm powers let sims throw around lightning but also lets them start and stop rain when they want, and cause a thunderstorm or stop it. Even when off the grid they can provide power with their mere presence. They can't get electrocuted. They can swim in a pool during a thunderstorm. They can also try CPR on a dead sim before the reaper has claimed them, hitting them with lightning to try and revive them which could have a 60% chance of working.
Transportation powers would be stuff with super speed as well as teleporting. Their bursts of speed would just skip animations though since I imagine otherwise having 8 sims doing everything at super speed would super crash the game. Like if you'd order a sim with super speed activated to eat a meal, you would not see them cook or eat it. You'd just see an empty plate on the place they would have eaten it and their hunger need is now fulfilled as much as the meal you wanted them to make would fill it. If the shower is free their hygiene fills when you click them to use it, not seen using it. They could however be more likely to break stuff or leave it dirty when using super speed, but they can clean it super fast. Not repairing though. They can teleport any unwanted sims away from the area they are in too, just not crucial NPC's like store clerks or such to not mess with the game or the grim reaper or such. But townies, trespassing animals, papparazi or such? Yes they could.
Should also be some sort of Kryptonite though that disables super sims powers and makes them weak. A rare metal, maybe one already in the game.
Could come with a new super hero and super villain freelance career, they'd just go out there, possibly flying off even, to go do hero or villain stuff which would not be steady hours. Sometimes an opportunity could pop up for villains to take advantage of, or heroes get an urgent thing they need to respond to. The time it takes would be random, so it could be challenging to juggle with family life.
Sims without super powers could also take up that career. The robotics skill would be expanded to make better exo suits that can do more and be more advanced, like iron Man. They could use tech to replicate various abilities of super sims, but can only do it when wearing the equipment. While they would not have the weakness to that kryptonite thing, their equipment can break, needs maintenance and such too and is a big investment in time, money and resources.