I'd like a new expansion pack and have some ideas.
1. Alien Planet. Yes they technically have one but we can't build lots on it and I think it would be a really cool and unique thing to do. They could have alien houses and special alien lots. They could have a special holiday on their planet where they have to abduct a human from another world and bring them back for experiments. I feel like there's a lot they could do with this.
2. Underwater Community. I'm thinking Atlantic for this first one because really wouldn't that be amazing? I love the mermaid expansion already but I feel like it can go deeper with this. They could make it where they lots are all underwater so only mermaid families could live in them. Also could make some dock lots for those who aren't occult mermaids in this world for starter lots and I think it could be really fun. They could include jobs like Guardians (Mermaids/Mermen who protect their world from humans) Hunters (Mermaids/Mermen who hunt for food in the sea) Mad Scientist (Trying to find the home of the Mermaid sims)
3. Mining World. Imagine if you will a world in sims with a great number of natural resources through mining. This could introduce jobs like miners, railroad workers, and factory workers. I'm thinking mountains, caves, and more.