Actually, the idea of a different exotic culture in each direction works lovely for what I'm seeing now.
I think I found my framing devices and inspirations:
1. the fantasy version of 1066 is the reason why things fell apart. a warlord from another kingdom (dubbed the bastard) is conquering a world. all nobles have either been cowed or slain. Seriously considering making him an avatar of the adversary and his knights essentially tieflings.
2. the peasants' rebellion of 1391 is my inspiration for the fighting itself. However, to give them a fighting chance, magical transmutation has occurred. Some peasants went to bed one day a human and woke up an orc (to be strong enough to go toe to toe with a knight). Likewise, city folks had a similar experience by waking up either a goblin or hobgoblin. They aren't evil, but forever changed from the experience and are now a visible "wolf's head."
We finally have a world where Gary Gygax's obsession with polearms makes sense.

3. This is going to be a world where medieval manuscripts define a world. Giant Snails bubble up from the earth causing mischief that low-level adventurers need to fight. The Vorpal Rabbit is a thing of great danger (as it was inspired by medieval tapestries of rabbits doing shenanigans). a buddy of mine got bored once and converted a lot of them over to 5e and now I got a world to use them.
Strange Monsters from History - Dungeon Masters Guild | Dungeon Masters Guild
4. The world is going to be mostly monotheistic, however, barely. Instead of worshiping radically different gods, it's more they have been really good at soliciting saints (time to pull out the big book of martyrs for this). The Fey and Daemons walk the earth and despite their nature, really don't like what's happening and are here to help. Elves are Fey that have lost their immortality and now have to learn to live in the mortal world.
edit: a couple of old threads I did in hindsight really add to my feel.
https://www.smithsonianmag.com/smart-news/why-were-medieval-knights-always-fighting-snails-1728888/ Okay, I'm asking, In medieval manuscripts, we have a surprising number of knights fighting giant snails. How did this great pestilence come to being in your dnd game? Were they a symbol of...
www.enworld.org
We've all seen the old joke where a town has to run for its life because a high-level adventurer needs a couple of experience points to get to the next level. Is it even possible to do a peasant's revolt in Dungeons and Dragons? Essentially how to gamify the following historical event...
www.enworld.org
How's that?