Well, considering your positive reaction to varied landscape, islands, linked portals, and lots of different and dangerous environments...
Have you considered a Shattered Reality?
Imagine if the Prime Material (or whatever you would call the core universe) was completely broken, shards of existence splitting off into a kaleidoscope of realities, each distorted, twisted, or broken in some fashion. The individual pieces link together, and you can jump from one reality to the next, but each has its own rules, environment, and metaphysical nature.
So each reality would function as a small community, city state, or minor nation, and could interact with the others in various ways. But due to the way existence has been distorted, ordinary creatures have become monstrous beasts, laws of nature have been turned upside down, and geography is a royal mess. This would give you a way to have lots of individual ideas all bound together, one shard with the mysterious fog, another with sweeping mountains and forest, a third in an inverted earth that is completely hollow, etc.
Set the timeline a significant time after whatever cataclysm broke the world, and you could make semi-stable societies that exist within a shattered multiverse, giving you plenty of space to make exciting destinations, but still have plenty of unexplored space for exotic adventures.