So, I got into D&D, specifically 3.5, somewhere around a year ago. Got all the books, read through them like a madman, then set out making a setting. Basically, Standard Fantasy Land set in Tasmania, and filled with enough cliches to make Tolkien blush. Then I thought, "Why not put a cool spin on this shit?" The campaign was axed, and the setting remade. Deities made no sense, the thought of a priest on every corner being able to raise the dead was ludicrous, and I didn't want players exploring an undeveloped world. So, all but one god was killed off, NPCs were capped at level 10, and Tasmania got stuck inside of a giant prismatic sphere to protect it from an interplanar war/apocalypse/ice age. A session or two later, then I thought, "Why not give NPCs common magic?"
Well, fuck. I axed the campaign, and remade the setting. Over and over and over, but nothing worked. I even made my own full-fledged planet, which was cool. On my quest to find a rational solution to what a magic-rich civilization would look like, I found the Tippyverse. In short, it's all about the extreme exploitation of the magic traps rules, giving you a world where fantasy tropes and common sense are bent over and fucked silly. Threw my hands up, figured magic needs to be so rare that the average peasant could die never having seen a spell cast, and lowered the NPC level cap even more. Then I found E6, which can pretty much be summed up as: "Want to go higher than 6th level? How about we bribe you with a bunch of feats, instead? It's totally more realistic!11!1!!!one"
Then I thought, "I don't want to bother with making a map. I'll just use North America, instead!" One thing led to another, and it became a post-apocalyptic world. Long story short, magic became real, massive EMP followed, almost everybody died, and lots of survivors got turned into elves, dwarves, orcs, and other shit. Spiderman was a trickster god, Mount Rushmore was a holy site, California was ruled by an angel-worshiping-magocratic-constitutional monarchy, and the other planes got fused together into a tidally-locked Earth-afterlife-thingy. Shit was cash, until I added guns, used math to find how many spellcasters there where, and realized that, after a few centuries, everyone should've become magitech-crafting sorcerers that could have daily chats with their gods over tea.
That's when everything went downhill really fast. Almost everyone became 1st level commoners, the NPC level cap was pulled down more, the population growth rate became that of the early Middle Ages, the gods were erased, planar travel was borderline impossible, all the cool shit like giants and dragons were killed, and magic was so rare that you could snugly fit all the world's spellcasters into a fucking football stadium. Then I thought, "What about long-lived races like elves and dwarves? Wouldn't they be dying faster than they reproduce?" Turns out most playable races live longer than humans! Fiddled with the math, and the world became nearly devoid of everything but kobolds, orcs, goblins, and humans. Scratch that. Kobolds took over, because they breed like fucking rats.