Honestly, except for major technological and rule differences, you can have pretty much anything all on the same planet, but in a different region/continent. A culture that has advanced enough to be able to travel the globe, however, would significantly impact every other culture, so they are best left to other planets or alternate planes (for a single use campaign, this could be an interesting idea to have the expansion enter the PCs region). A spacefaring race could be on another planet, with limited interaction with the main planet, but then you get into sci-fi, which you wanted to avoid.
Unless you have a very specific reason why the laws of the universe work differently in a region, that's going to need to be another plane of existence. Even if you have a cool reason for something, you really shouldn't pull it off more than once, unless you expect such radical changes to be the norm for the entire world. Some players might enjoy Weirdworld, but I imagine the novelty would wear off quickly for many.
Unless you have a very specific reason why the laws of the universe work differently in a region, that's going to need to be another plane of existence. Even if you have a cool reason for something, you really shouldn't pull it off more than once, unless you expect such radical changes to be the norm for the entire world. Some players might enjoy Weirdworld, but I imagine the novelty would wear off quickly for many.
