I wrote this in another discussion, but I think it would be helpful here. I apologize if anything is repeated.
The merfolk would be better if you could explore underwater and had a level system similar to celebrities, vampires, and spellcasters.
Don’t get me wrong, weather manipulation, moodlet manipulation, the siren call, summon ocean threat, and aquatic lure are all great abilities. However, you could easily break the abilities into groups and make them stronger. Ocean Mastery, Sea Shanty Mastery (Or Siren Mastery even though Sirens and Merfolk are separate but similar mythical creatures), and Weather Mastery could be a nice breakdown. Abilities based on reputation could be included.
Ocean Mastery could focus on things like the Aquatic Lure, Fishing, and Summon Ocean Threat. I do know of a hidden trait that makes traversing the ocean in a boat easier. Maybe it could also remove the sad moodlet from eating and catching fish.
The Siren tier could focus on the Siren’s Call, Siren’s Lullaby, and the singing skill. The singing skill and other musical skills could be improved faster. Things like singing Karaoke could have unique moodlets and impact other sims more.
Weather Mastery could impact how often a sim can change the weather. Instead of just summoning Rain, a Thunderstorm, and Clear Skies, a sim can summon more unique weather patterns, summon lightning like when you get supercharged and summon wind. Both wind and rain can be very helpful for building skills and maintaining different things like a garden. Maybe a mersim with a bad reputation can summon unique weather like Monsoons and Heat Waves.
Abilities would still tap into hydration to use and passive perks might affect this. Weaknesses could be a tendency to dry out faster, require spending time in the ocean (Changing into merform and interacting with sea creatures similar to the Azure Dolphin relationship bar), having a song affect a sim in an unintended way if certain conditions are met.
I can see this being added to the game with an expansion pack similar to the Jungle and Outdoors one. There could be a vacation destination centered around the sea, diving, sailing, and music. This place could be an extension of Sulani, and an option to follow your sim underwater could be made available.
One last edit, I understand that movement underwater can be a challenge. Kingdom Hearts 1: Atlantica and Neverland had a lot of controversy over the controls. I actually picked up on the controls intuitively and enjoyed the gameplay. I think with the right user testing, the Sims can include something similar or better.
You know how there’s a sage for the different spellcaster schools. It would be cool if merfolk could discover a place that can allow them to enhance their mermadic abilities via the Ocean, Siren, and Weather Mastery. Merfolk that choose a school might have more abilities via the options from their natural abilities.
Merfolk with Ocean Mastery might be able to fish and eat fish without negative moodlets, build relationships with more unique marine life they summon like the Azure Dolphin, and they can gain a boost to the fishing skill as well as navigate the ocean with ease.
The Siren Mastery could unlock songs that can induce more moodlets. Right now, a mermaid can only make other sims sad, tense, flirty, and inspired. There’s the focused, angry, embarrassed, confident, playful, bored, and different variations with different extremes. A passive boost to the singing skill with unique moodlets sims experience when playing an instrument, singing, or listening to songs or played music.
Weather Mastery could cause snow in the summer, summon lightning strikes (useful for collectibles and it’s cool), cause a heat wave in the winter, summon wind, and location unique weather anywhere like monsoons.
