Islands World.
No landmass larger than New Guinea. A world of a million islands.
Humans are a rarely seen folk, as are high elves, and one or two other common core species.
IRL touch stones are Mediterranean cultures (especially heavily diversified ones), Pacific Islanders, and Celtic And Norse cultures, and American indigenous cultures. Cultures tend to be mixed ancestry, with actual robust rules for mixing species.
The tropes are exploration, magic and tech working together, and living in an age where people are mostly thriving and totally free of the “the old world was better” trope.
The aesthetic is Final Fantasy meets studio Ghibli meets Zelda: Breath of The Wild.
9 shows up a lot. 9 moons, 9 “continents” (the largest islands, always at the center of archipelagos), 9 Celestial Houses in the sky representing 9 divine courts, 9 Great Lords Anathema, and 9 Dragons of Light and their 9 Dragon Temples.
The gods are present in the world, and when a person really strongly emulates a god and thier stories that god can basically become embodied in that person for a short time, guiding them to even greater feats than they’d normally be capable of. No evil gods, just gods and their enemies the Lords Anathema which are aberrations and fiends, mostly.
Aether infuses the world and runs a lot of the magitech. It can also be captured, compressed into crystalline residuum which can be used in place of costly material components.
Larger Aether crystals can store energy and redirect it, powering things like airships and other large engine devices. Such devices can have attunement slots like a character, allowing customization via installation of specialized systems.
New classe: Aethernaut- a swordmage style class with a pool of dice used to fuel arcane techniques themed on infusion of the body with magic energy and bonding a weapon. Lots of teleportation and weird movement (turn into lightning and end the turn in the destination space ofthe line of lightning you became). Has moves like throwing the weapon and it explodes dealing attack damage to everything in 10ft radius from a point you choose.
Artificer subclass: Artificer, Aeronaut. Vehicle focused artificer with a special gun and a familiar that can possess a vehicle and make it sentient for a time. Possessed vehicle can bear extra systems. Can install special gun into vehicle giving a vehicle a specialized magitech gun.
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Big campaign seed:
The Dragons Awaken
After a thousand years or more, one of the dragon temples rises from its resting place, reminding the world that these temples are sentient divine constructs, and it has been corrupted by Anathema. Now heroes must rise to awaken the other temples before they can be corrupted, and entire into the corrupted temples to cleanse and restore them.
Provide stats for 5-6 temples, corrupt and not, with stats for fighting the temple itself, and stats for its defenses once inside, and regional effect stats like the regional magical hazards in Tashas.
Maybe include the ability to earn the companionship of Temple Guardians, which are Large - Huge draconic constructs with that are basically sentient magic item vehicles. Maybe a whole thing where when the party had restored its first temple, they get fairly limited guardians, just flying mounts that can barely fight but are hard to kill. Each successive temple them restores the guardian’s power and personality.
Less grand seeds include The Great Hunt, where a young woman in a remote island has sounded The Horn, thus calling The Hunt. Heroes from across the world gather to prove themselves in trials and gain a blessing from the horn, and then disperse again in search of the Arms of Fate to better combat the Great Beasts that the horn was sounded as a response to.
Regardless, a fairly peaceful and fruitful world is threatened by a half forgotten darkness, and heroes must rise to meet the darkness and defeat it.
Another possible new mechanic could be Fellowship mechanics involving group morale as you form bonds doing adventures, and bonds with communities as you help them, and this translates into greater Resolve, Hope, etc. Resolve Luke be a sort of group defense mechanic, while Hope is a dice pool like Force Points in Star Wars Saga Edition.
Definitely build on existing mechanics for investigation, reputation, hazards, and expand encounter building to make the enemy both creature and environmental hazard. You have to engage with a temple or Great Beast on multiple levels, think in terms of combat, and problem solving, to win.
Airships are a newer tech, and regular folks mostly have access to small personal short range craft, which needs to land after an hour or so in the air and recharge. You need a ship early on, and only in like twice 3 do you possibly get something like a long range airship.
Maybe there are floating sky islands, as well as normal islands.
High level play unlocks access to the Nine Moons, which house celestial secrets of epic importance and power, some of which will try to eat you, while others will “gift” you with some terrible responsibility.
This gives three physical tiers of focus, not counting the Depths of the Eternal Sea.
Some worldbuilding is left up to the group, by way of mechanics for players and DM to decide where each PC comes from, what species are common there, and what life is like there.
As well, major cities, hidden places, and whole nations, might have options to either stick with a default “canon” or turn worldbuilding back to the players with a sort of mini-game with multiple choice options.
Also include rules for determining the legacy of a group of characters, giving an easy way to play the next story in the same world but years later with things changed by the heroes of the last game.
Also this would be a “some or all of this could build a fantastic setting” pitch. A lot of it is optional.