I'm thinking of starting a desert campaign for my players, but I can't really decide what mythos I want it based in...

• Egyptian-Gods and Goddesses play a minor role in everyday life, but are constantly struggling betwixt themselves, leading their religious sects to war with each other.
• Babylonian-Gods and Goddesses play a direct role in everyday life, constantly fight, and ritual sacrifice on a daily basis may lead the PCs into unpleasant situations.
• Assyrian-people worship the Gods and Goddesses more out of fear of their wrath than true devotion. They don't impact daily life much, until they fight, which could destroy the world.

These are the three that I roughly know the ins and outs of. Any ideas or insights into the cultures would be appreciated.

Personal suggestion.  Take all three and make an amalgam of the three mythos' you refer to.  Those religions shared many similar stories (although that can be said of almost any mythos) so perhaps you can use those as the framework to make something original.  Even if it is something as simple as you yourself just... reskinning some of the gods then do so.  Make something new and novel, something the players might feel slightly familiar with but when they start becoming a little confortable in where the stories are going throw in something they are not familiar with.  I mean how would Gilgamesh interact with Amun-Ra?  Tiamat (mesopotamia version) dealing with Zarathustra (zoroastrianism) and creating disasters because of the conflict.  Change the names but perhaps keep some of the stories the same.  This allows the players to use some of their real life knowledge, while not making every interaction a foregone conclusion.

As odd as it sounds I have been thinking of ideas along these lines because of the old Xena and Hercules TV shows which ended up exploring a number of different pantheons and the ...mix and match ended up making for some very entertaining events.  

If you really want to stay to one pantheon I would say Egypt.  It is something that many people would have surface knowledge of but are unlikely to have in depth knowledge of some of the stories.  For example the dissolution of Osiris and/or the reformation.  That in and of itself could be a very interesting game story to play through.

I wish you the best of luck in your game.

Quote from HeroZero >>

As odd as it sounds I have been thinking of ideas along these lines because of the old Xena and Hercules TV shows which ended up exploring a number of different pantheons and the ...mix and match ended up making for some very entertaining events. 

Hmmm...I must say that does seem to have the most promise for a fantasy epic. I may start work on this. I have a dark fantasy story going on right now, but my heart isn't really in it anymore. It's to linear for my type of story. I like giving my players a sandbox type campaign and that's a story driven setting. Maybe, there's a sun worshipping cult in the starter city and a crazed, sun baked wanderer staggers through the city gates raving about the death of the sun. And then it eclipses. The priests pray and the sun clerics perform several sacrifices, but it remains darkened.

Quote from DonJonMastah >>

Hmmm...I must say that does seem to have the most promise for a fantasy epic. I may start work on this. I have a dark fantasy story going on right now, but my heart isn't really in it anymore. It's to linear for my type of story. I like giving my players a sandbox type campaign and that's a story driven setting. Maybe, there's a sun worshipping cult in the starter city and a crazed, sun baked wanderer staggers through the city gates raving about the death of the sun. And then it eclipses. The priests pray and the sun clerics perform several sacrifices, but it remains darkened.

As a bit of a side note, if you want to pick up a bit of... surface knowledge of various gods (with a humorous presentation) might I suggest Godchecker.com?  

http://www.godchecker.com/pantheon/mesopotamian-mythology.php?deity=NERGAL

is a good example.  I have found it a good place to start and get ideas.

Haha it sounds like you guys are as in to this idea as I am. I just might have to post that campaign on here when I get it drafted up. To answer your question; when the sun turns completely dark, the ground quakes violently as ominous black and red clouds swirl overhead. A woman's cry is heard from in the midst of the onlookers. As the crowd parts around her, we see a large splat of blood on her face. She starts sobbing as the village gets pelted with a violent rain of blood. The wanderer falls to the ground and starts muttering prayers in a strange tongue. With everyone running through the streets to escape the deluge, the city descends into chaos. A few unfortunate souls look back to the sky only to see the silhouettes of two titanic figures locked in a struggle on the clouds. Venahura the king of the underworld has come back to claim his vengeance against Maht Hasheff the sun god, for killing and banishing him to the underworld. As lightning flashes, winged figures can be seen clashing in the clouds. 

I'm thinking of making the pc's vessels of godly power. Not quite demigods, or as strong. But not letting them know it until say, a jrpg style unbeatable first boss.

I like, I like.  Sounds like a good apocalypse on the horizon.  

Indubitably

Since I'm making a scramble of a pantheon, I thought I'd bounce some deity ideas off of you. 

1. Maht Hasheff-sun god and king of the Burning Sands Gods(4 armed and winged humanoid)
2. Venahura-Lord of the underworld(has 100 arms,humanoid)
3. Inek-goddess of love(human looking)
4. Cravah Magya-falcon headed god of war
5. Kopoh-a 10000 foot long cobra, said to be the keeper of eternal wisdom
6. Sihbala-waifu of Maht Hasheff, protector of mothers(maned lioness)
7. Great worms-not really deities, but worshipped by some nomadic tribes
8. Hyla Xephra-cannibalistic demoness, wife of Venahura
9. Aphraeset-fire haired god of smiths, Sihbala's brother
10. Maha Runa-obscure warrior goddess, actual creator of the world(via the slaying of Maraso)
11. Maraso-titanic winged cobra, lord of destruction(slain by Maha Runa and then torn asunder to create the world)
12. Volegra-massive demon, representative of mans eternal hunger for more than he has(convinced Venahura and Hyla Xephra to slay Maha Runa)
13. Bassabbi-giant golden cat(has wings) guardian of tombs and shepherd of the dead to the underworld(also, he's a cat, so don't make him angry) 0__0

I also have a couple of high priests made up.

Mabrossa is the high priest of Maht Hasheff and a human, though he wields the power of a high priest, the prophet's arrival and the subsequent chaos have made him start to doubt his faith.

Marbas is the high priest of Venahura and half tiefling, half earth genasi. He harnesses the power of the death domain and is the Chosen of Venahura, giving him access to even more power.

I like what you have started so far.  Next thing that will likely be important is creating some basic relationships between the gods.  Whether you flesh out why they have those relations or not is something you can work on later.   Who is likely to be on which side in this conflict is going to be fun to come up with, and I would suggest having a few "fence sitters" that have not chosen a side yet (or are entirely neutral) and maybe ones that might be willing to switch sides if it looks like things are going against them.  Advanced idea would be maybe some of the gods see this as an opportunity to get even, when everything is chaotic there will be less interference if they should try to knock their half-brother off the pantheon.  Permanently.

Good Luck and have fun.

I was thinking of making the more "aware" deities more likely to be neutral, Kopoh for instance, knows that their conflict will destroy the world, but the others are too hot-headed to listen to what Venahura calls, "the blathering of an old serpent." I also have a few more of the chosen made up.

1. Nehrahka-A member of the Nahro tribe, she and her people have always revered the great worms of the desert. In exchange for food and worship, the great worms allow the tribesman to ride on their backs, or just don't notice them. Chosen of Kopoh for he created the great worms by shedding his skins.
2. Acktiba-Dunestrider(anthropomorphic cat people, cat like attributes make them look like half human, half sphinx cat. Chosen of Bassabbi for it was he that first created their race.
3. Gale-aarakocra monk and chosen of Cravah Magya (according to this world's lore, He created them)
4. Calladiya-Sand elf priestess of Inek. Hybrid wizard/sorceror.

Sihbala, Aphraeset, and Maha Runa do not have chosen. Sihbala is always assumed to side with Maht Hasheff. Aphraeset is the only god that is medium sized, therefore does not need an avatar on the mortal plane. Maha Runa is just dead and therefor has no power to bestow upon a champion even if she could choose one.

As for the demons; they are capable of possessing a mortal of suitable power, IE minor demons can only possess mundane creatures, lessers can inhabit mundane creatures OR mortals, demonic adepts are mortal priests of their demons cult that have bonded with a parasitic demon that slowly consumes their soul in exchange for dark powers or transformation into a Phane Guardian. (a more magic based blackguard) Elite demons are the leaders of the underworld's legions and are all considered "boss" monsters. Finally the Lords of Hell and the Court of the Three. The Lords of Hell are all monsters that rival minor gods in strength and embody the base desires that are the roots of all evil. The Court of the Three are Venahura, Hyla Xephra, and Volegra.

Only Venahura and Maht Hasheff know the prohpecy about Maraso coming back in the end of days. Kopoh himself was told this by Maha Runa before she was slain. Consequently, only Kopoh knows of Maha Runa's return to balance the scale between good and evil. She's going to kill all of the other gods so that there is no more conflict between good and evil. The cost? Universal magical death, when the gods die, the magic that they brought into the world dies also. Magically and spiritually attuned mortals will die, magical creatures will die, Paradise was wrought with magic as was the underworld. She's going to destroy it all to rebalance the universe.

If my players like the region, I'm going to implement a contingency for continuing the story in other lands. The Burning Sands is the central country in a small continent called Menalaen. The religion of this world was was once used worldwide. It is still in practice, but the Burning Sands gods are dual faceted and the other aspects of the gods are worshiped in other countries. For example, Maht Hasheff tolerates people worshiping the other gods, but hates demon worshipers, however in Vrodstadt (the country on the south east border of the Sea of Burning Sands) His worship is the only publicly accepted religion, any other religion's priest is likely to be chased out of town with torches and pitchforks. After their church is burned down, and the priest burned at the stake as a heretic. Dwarves are the main protagonists besides PC's in the Eradhell Mountains, therefor, they mainly worship Aphraeset as the smith of the gods. Out in the frozen wastes ad highlands of Vollheim, its variant humans (I think I shall call them the Osoba its serbian for man) predominantly worship Maht Hasheff's darker aspect-Baelor the war god.