I'll start off by saying that the following is largely made up of the way current campaign setting project
was created and the thought processes behind it. I have to say though, that because I worked with ideas other people gave me, I cannot always elaborate on some of the thought processes, as they aren't my own.
The very first thing we did? We took the real world and threw concepts at it taken from various video games, anime series and the occasional fantasy novel. Then I sat down thinking, "hm, this is a pretty nifty world so far, but it looks like too much of just a mash-up so far, plus I'd rather have original names and concepts."
From that point, we made up some new names for the continents to replace the old "Europe", "Asia", etcetera, while I came up with new areas for the world to give it a bit of a new look and new geography. It was mostly just "put various biodomes wherever there still aren't enough yet" and thus ended up with a jungle in what was once called North America. With more ideas thrown in later, we ended up with a more tropical version of North America (renamed Zailleon), a colder South America (renamed Cher Plek) and an Africa that was nearly all desert (renamed Kemei). Furthermore, because I had recently read A Game of Thrones and liked the whole "giant wall to keep the monsters out" idea, I put one in the north of Asia (renamed Kouchi), beyond which lay intensely cold lands, with the North Pole being occupied by a mountain that was so huge it could be seen from outer space, as if the planet were wearing a party hat. Of course, the size of the mountain was later downscaled and it was moved to Antarctica (renamed Cyra) to make place for something different, namely an eternal polar storm. This made it so that at least Cyra had some more stuff going on there, as it has always been quite empty. Finally, because we had settled on magic having elemental themes, I gave each of the continents an ancient "temple" that was a nexus of magical power for one of the elements. Their placement was largely random, except for Water (which was placed in Australia, renamed Valtheran) and Ice (which was placed in Cyra).
Considering I've now touched upon the theme of magic, I'll get further into that. I finalized the settlement on elemental themes for magic by making a bunch of "spheres" of magic, each with their own theme, which was one of the elements, and gave them all a number of spells of various strengths. A number of spell ideas fell outside of the elemental theme, as we couldn't think of any element to place them in, such as necromancy and divination, so we kept them seperate at the time. Later revisitation (after I started playing DnD, as the setting was first created before I knew of DnD) learned me that I could make some elementally-themed spells for both necromancy and divination, with spell ideas like "keep a corpse electrified to create undead", "stealing someone's shadow to control them", "scrying through fire", "scrying through water", and so on. It became even better once we started thinking about how the world came to be, as we wanted an original origin myth, and from that came even more ideas for elementally-themed spells you might otherwise not think of, such as ice-themed time spells, lightning-themed enchantments, fire-themed healing spells and water-themed abjurations. We clearly went with the "high magic" approach way back when, but this was because we considered things in a video game style setting with a very gamist point of view, whereas the recreation of it is meant to be at least a little more simulationist, as befitting of DnD. As such, the campaign setting generally has moderate amounts of magic, but there are both low- and high-leveled uses of it around.
One thing that also helped in keeping the magic in check a little, or at least not be needed in a lot of places to justify things, was the inclusion of technology. As of now, it's still a little hazy on what level of technology the world will be at, but right now the idea is that they are at least at 1800s levels, with the Holy Empire of Stamaria and the Angels being at higher levels, due to greater push for progress. It isn't just magitechnology, but just straight-up, real-world stuff, such as electronics and machines. The idea to use magic as a power source for those is an idea however, making it subtly into magitechnology again.
I have more on other subjects, but I'll just post that some other time.
(Also, people are actually free to post in my campaign setting thread. Some people who've talked to me about it apparently think they aren't allowed to, or shouldn't, for some reason. But really, you should!)