The first thing you need to decide is the kind of world that you want to create. To my knowledge, there are 4 major types of worlds and countless minor ones. The 4 major types are the planetary system, infinite plains, islands in the sky, and unending caves. These are just my names for them and are not necessarily shared by other people. If you have suggestions for additional world types to add, then please say so in the comments. If you provide a decent description of it, I’ll even put it in word for word. Regardless, I will mention that you suggested it first.

PLANETARY SYSTEM

This is basically just how it sounds, the world is a planet, it exists in space, it orbits around a star, etc. It might also be possible for interplanetary travel to occur, though it is not necessary. The main benefit to this method is that there is a considerable limit on how much backstory is required, as a planet has a rather limited surface area. That is simultaneously it’s main inhibition as you can only expand so much before you cover everything. Often interplanetary travel will necessitate sci-fi elements, which will be entirely up to you.

INFINITE PLANES

This, too, is pretty much exactly what it sounds like. The world is an “infinite” plane. What this means is that, typically, while you technically could travel an infinite distance, after you travel a certain distance from the center, either the conditions become inhospitable, or magic and physics become warped to dangerous, unpredictable levels. There are typically numerous planes that can be accessed with relative ease, typically by portal, and each plane’s environment is usually completely different from that of the others. This template makes it ridiculously easy to do world expansions, as they can simply “discover” the portal coordinates of a new plane. However, you have to be pretty creative in order to give each plane its own, unique appeal.

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THE ISLANDS IN THE SKY

Okay, let me be honest here. All of them are almost exactly what they sound like. Basically, there are endless floating islands, with any size imaginable. There is often a scarcity of resources, as anything that falls off the island never returns. However, the islands can be any size, so any that are large enough can have hundreds of oceans, and some of the inhabitants might not even realize that they are on a floating island. Often, islands will appear in clusters of a similar type, such as a jungle cluster, or a mountain cluster.

UNENDING CAVES

You probably know what this is. It's… the unending caves. Unlike infinite planes, the caves can go on forever without laws of physics and magic breaking down and are very hard to put in a book. You can, of course, make it like the infinite planes so that the laws of magic and physics deteriorate dangerously the further you go. The main limit is a lack of food production facilities, due to being underground, though it could be combined with the Infinite plains template to allow for the importation of food and water, along with exportation of resources.