These are likely HUGE pains to code, and would likely be better as DLC/expansions ala the Sims series. Still, I think they'd be cool. Visiting Zuzu City Again: Idea that this would be an unlockable option with either the bus or with marrying Sebastian or Sam. Of course, programming an entire megacityscape would likely be too much of a coding pain, but maybe stuff like a unique bar/restaurant? The club Sam's band plays? A clothing shop with unique clothes that aren't available in the starting menu? A store that sells home goods not in the catalog/at Robin's? A hotel that allows you to stay a day? A new marriageable NPC? The possibilities are endless, and it would make a cool expansion or simply an awesome update. Desert Farm and Desert Town (possibly better as expansions): As you know, I mentioned the idea of the Desert Farm/Ranch in my last thread as a cool yet challenging idea. A farm where growing crops outside of a greenhouse is almost impossible except for a few select kinds that can grow in the desert, more reliant on animals, mining, foraging in other parts of the map, or other options. I'd suggest adding to this with a Western/desert type town to add to Sandy's place and the Casino - make those the edge of it, and make an expansion out of this - a New Game + where you get to build more than a farm once you've accomplished the "endgame" milestones - you can set up, build, and populate a desert outpost to turn it into a thriving frontier town.
Zuzu City could be opened up late-game by another bus expansion (larger fuel tank, got lost somewhere, go find it) and be a reliable source of more modern stuff like battery packs or something. To keep it small, it could just be the mall with a few shops. Still, a nice addition I might say.