I have decided to write my own homebrew campaign to run my campaigns in.  I wish to reduce the amount of meta-gaming by not using other settings.

My geography will consist of the peninsula of Italy, mirrored left to right.  By doing this my geography will be sensible instead of strange.  I intend to locate all the cities in exactly the same place they exist roughly today.  I will change the names of all the cities and settlements to fit the race and to obscure the fact that I am using Italy.  So there won't be a Venice or Rome of Sicily, but some fantasy name depending on whether the area is settled by Orcs, humans, Elves or Dwarves.  These will be my "civilized" races in the game.  I will also have regions of goblins, hobgoblins, gnolls, bugbears and even nomadic tribes of half-orcs.  I'm sure I will allow players to be any PHB race so we'll have to work out where the tieflings and dragonborn come from.  Halflings, gnomes, half-elves, and all will be just small cultures spread throughout human and elven lands.

The Elves will be first on the scene having sailed here across a wide ocean on the western coast (which on my map will be from the direction of France because of the mirror effect).  The Dwarves have spread into the area from the mountains in the north and generally confine themselves to the mountains through the center of the peninsula.  The humans will have sailed here across a narrow sea to settle on the east coast (having crossed the Adriatic).  The Orcs will have sailed up from the south to settle in the southern most reaches of the peninsula.

The Elves have no desire to mine the mountains and are willing to trade for the goods they need from the mines.  The Dwarves prefer to stay in the mountains but need a little open land to grow crops and livestock.  They also have cultivated groves of trees to obtain their wood resources, because they are very uncomfortable in "natural forests."  Humans are tolerated in the mountains by the Dwarves provided they don't interfere with the areas already claimed by the Dwarves.  The Orcs, being an openly militaristic culture, don't much tolerate anybody, but at the frontiers they don't always have the means to impose their will on any of the other three.  The other three will quickly drop their petty differences to fight the Orcs.

The half-orcs fled a general persecution in front the advancing Orc and Human cultures.  They behave generally as the Native Americans in nomadic tribal cultures that travel the lands of the hills near the mountains.  They are a "horse culture."  There are small villages and hamlets of halflings all along the same reaches; the hills near the mountains.  They have existed for very long but their written history doesn't cover much detail.  Also the halfling settlements exist independent of one another so there is no "halfling nation" contending for control of the peninsula.  There are two distinct half-elf settlements in the north, one on either side of the mountains.  Other than those, half-elves are derived in the usual manner of the mating of elves and humans.  But the two half-elf cities are different, because they have a "national identity".  These two cities have a long history and hold more knowledge of the ancient history of the peninsula than any other place.

Archeologists have uncovered evidence of ancient settlements.  They haven't been able to piece together what happened and the two half-elf cultures don't want to share what they know.  However, it is understood that there was a time when there were cities and towns all over the peninsula as you would expect.  Something killed all that off except for the halflings and the two half-elf societies.  The goblins, etc., trickled in before the half-orcs fled here ahead of the humans and Orcs.  Other fantastic creatures like giants, trolls and such are believed to live here from ages past.  Then the Elves arrived first to settle and tame the land.  The Dwarves advanced from the north staying close to the mountains.  The humans and Orcs arrived about the same time and now all these cultures are beginning to contend with one another.

Each have their own governmental structure.  The Orcs are led by a dictator, something like a super warlord.  The Elves are led by three emperors that sit in council and vote on national affairs, but run their own provinces locally.  These emperors are served by a Grand Council collectively, but have their own Council for their provincial affairs.  The Dwarves are led by a King, who is elected by the Dwarven barons.  The king serves as king for the rest of his life or until he steps down.  But the affairs of the communities are run by the Guilds in the dwarven communities.  The humans have the most difficult political structure.  First, they still answer to their King in the Old Country.  The king has appointed two barons, one to the north and one to the south, that run the provinces separately.  They in turn appoint Counts, Dukes, etc. to run counties, who appoint noblemen to run smaller regions.  But with two political leaders there is sometimes squabbling between the Barons and the King has decided to let the Admiral settle their disputes in many cases.

In this setting I will write the story.  The theme of the story is Leviathan.  It seems there is an underground organization or a cult or something known as Leviathan that is up to mischief on the peninsula.  The party will first settle a few small matters as they gain experience.  Along the way they will hear about, and collect some tokens, Leviathan.  From level 5 to 10 they will navigate a series of challenges to face either a dragon of wyvern and be led to think that they have overcome Leviathan.  Upon leaving that encounter they will have a vision in the skies that they have not overcome Leviathan, but instead made themselves key targets of Leviathan.  Ultimately they will conclude their battle with Leviathan on one of the other planes or something.

For now, I plan to flesh out some details about the peninsula and name the cities.  I'll have to decide which are towns and such and where the roads will be since this will be a much younger Italy than today.  I'll also have to skew it for the two half-elven cities up north.

I'm sure I'll post some more about this as I develop it.  If you have any observations please feel free to share.

One other dimension to this world building is the division of pantheons.  In history I learned that each significant ancient culture had its own pantheon.  Since we are theorizing a number of ancient cultures rubbing up against one another, it seems right that each should have their own pantheon.  I figure to give the human cultures the Greek pantheon as their default set.  The dwarven groups will be disposed to the Norse pantheon.  The elvish groups will typically follow the Roman pantheon.  The Orc culture will follow the Egyptian pantheon, unless I can find something better.  I think the two half-elf cities will follow a far east pantheon (and at least one of these cities will be a samurai culture).  The halflings will not have a pantheon but will have three or four ancient gods they follow, mostly agricultural and nature.  The half-orcs will be druidic or borrow from the human gods.  Many of the evil forces will be worshipers of elemental forces as well as the occasional wizard, sorcerer or warlock.

Let me know if you have any good tricks that tie into the different pantheons used.

Remember that roman and greek pantheons are quite interchangeable - the main difference is the names (like Zeus=Jupiter, Hera=Juno etc).

I think Egyptian for orcs sounds a little strange. I would have gone for perhaps slavic or gaelic if you want to stay in the Mediterranean area. You could also take more shamanistic approach like lappish or some sub-Saharan tradition if you want something less "civilized".

Looking further into the geography of the campaign, I believe the whole of the campaign may be situated in the provinces of Lazio (area including Rome), Umbria, Marche, Abruzzo, Molise, Campania and Apulia.  The Elves will be in the coast regions of Lazio and Campania, the Humans will be in coastal regions of Marche, Abruzzo and Molise.  The Dwarves will be in the mountains between the two.  The northern most portion of Apulia will be "disputed territory" between the Orcs and Humans, as will the Southern most portions of Campania be "disputed territory" between the Orcs and Elves.  A very important trade route will begin in Pescara wind thru Abruzzo, cross thru Lazio, touch Umbria and thru Terni and bend back to the left as it winds its way back to Rome.  This will be the road used by the Elves and humans to conduct trade and diplomacy with each other and the Dwarves.

The three elven Imperats will have a province to lead (each); Tuscany, Lazio and Campania.

The two human Barons will have areas around their two primary cities; Bologna and Pescara.

The Dwarven King will be up in the north, far away from the action but one of the Dwarven (Barons) Dwaeguards will be in the mountain portion below Terni and a second one will be located near Terni.

The Orc Dictator will likewise be far from the action in the south but he will have Warlords that will be key NPCs in the disputed regions.

I am developing the map, noting the extent of the mountains, location of rivers and towns (cities) all through this area.  Next I will have to name each settlement and prepare a synopsis of the demographics of each.  Then I am going to develop a sketch of the economy that makes sense to support the population.  A few Dwarven agricultural settlements will have to be dropped in to provide for the dwarves, but a large portion of their diet will come from mushrooms and fantastic beasts that live underground.  It will be a challenge but I will need to map out the Dwarven tunnel network to some extent and then develop generic Dwarven settlement diagrams for the hollowed out cave cities.