I like another desert world. The green worlds are very dominant in quantity at the moment. I also don't mind it being an American world for this particular theme. I always thought either an American world or a historical (medieval) setting would go well with horses.
In general however I prefer a bit more generic worlds. I fondly remember the sims 2 with more neutral worlds where we as the player could decide what to do with the. Of course the fact that we could edit the world, including the choice between green/desert ground, helped a lot. In sims 4 it is the devs that decide the worlds without giving us a say in it. The worlds are pretty enough, just the flexibility to adept them to our needs is missing.