I would love to be able to have a Create-A-World where we can edit thinks off the lots without having to use a mod. Twisted-Mexi made some mods that are helpful so we know that these things can be done. I want to be able to make mountains or flatten terrain and create larger lots.
Editing because I forgot to mention that I wish we could visit & build on areas that are usually hidden unless you take your sim there. Like the Forgotten Grotto, vacation spots, workplaces that we follow sims to. Thus far I've only seen the science career lot (from Get to Work) because I've only ever had scientist characters. I want to see the layout of the other places without having to take a Sim there. I also want to edit alien worlds. I'd love to be able to create custom alien worlds that my sims can visit when they travel via teleporter or rocket. It would be kind of cool to have Stargates (but maybe change the name to avoid copyright infringement stuff) and sims can go to different worlds through that. I'd love for sims to be able to build homes on alien worlds and live there and find jobs or take a teleporter to work back in simland.
I also miss the transporting via underwater portals. Remember how we could have a pool of water somewhere and sims could dive in and go to a different location in Sims3? I want that in Sims4. That ties in to entering an underwater world. Maybe there can be a career where sims have access to a submarine that works similar to the rocket and the submarine can take them to a Mermaid area (as was already mentioned). Sims can wear dive suits and have helmets and oxygen tanks. They have to leave after a certain amount of time to refill air or the Mermaids can give or sell them something that allows underwater breathing without oxygen tanks.
I would also like a world in a bubble underwater for non-mermaids to live.
I want to be able to set spawn points for things like we could in Sims3.
I would also like to be able to switch between households without losing anything. In previous games we were able to switch from one household to another easily.
Another thing I'd like for building is to have a Plumbob the Builder character that can be used to "inspect" lots to make sure everything works as it should and that there are no routing issues with objects.
I also second the requests for mass eviction, leaving furniture behind when family moves out, etc. A clone world option where you can copy the neighborhood, rename it, and then alter it would be fantastic. Oh, and the weather in different areas-- maybe some are always cold (as someone suggested in another thread-- endless winter) and some always dry & sunny. Others more tropical and never getting cold but having lots of rain (more tropical).
I would also love to be able to build custom apartment buildings and cram more families in. There can be settings for apartments such as "no pets allowed" and "no children allowed".