I would love to see the option to be able to create your own world. If nothing else, it would be nice to at least have what are essentially blank canvases that we can customize sort of like in Sims 2 and I believe Sims 1 as well. I really miss being able to place my own empty lots, and I don't really like being stuck with the pre-existing lot sizes that are already in the game. I miss how in earlier games you could not only place your own empty lots, but then place decorations outside of them and make them more to your liking. Newcrest is nice and all since it is nothing but empty lots, but you're limited to only being able to customize what actually goes on those empty lots.
If there's one thing I really hate about Sims 4 other than all of it's bugs and glitches and the absurd number of packs, it's the fact that so many customization options from previous games are gone. For example I really miss the option in Sims 3 (Create A Style) to give certain objects new textures and/or colors and really match things to have similar looks. Heck even with just the base game having that option made it really nice because then you could potentially give the limited number of available objects a different look, and end up with a large number of different visual combinations giving you the illusion that you have more objects available than you actually have.
Just imagine the amount of customization you could potentially do with just the ability to create your own world as well as a Create A Style feature. If they were to add stuff like that into the base game I would love that, especially as someone that only plays the base game.