The campaign will feature much planes and sphere hopping. It will involve a radiant dragon, dragon sorcerer king, vampire lord, illithid savant and the twisted ancient history of the neogi and drow.
Thanks to Issiah on OSG I think the main villain will be Mephistopheles. He rules only one layer of Hell but would love to kill Asmodeus and rule all nine.During one of the battles of the Blood War, one of his generals returned with a piece of the Eternity Puzzle. Mephistopheles, knowing what the artifact was, set himself to getting the rest of it in order to create his own Hell, rule how he wants and perhaps use as a place to get ready for a war against Asmodeus. In his attempts to take the puzzle, to not be noticed, he would use possessed mortals instead of devils.
Corelleon Laretheaon through spies in the ranks of the devil's armies also knows of the plans and is going to help the adventurers along as a beneficial aid sending different emissaries in various forms. Barkeeps, a frog, a sentient stalk of broccoli maybe to help the party when they need it most. Toward the end of the game he will have his radiant dragon friend try to make contact with the party as they encounter the legendary Spelljammer and a party of Neogi Slavers.
At the advice from Brian from OSG I will be using some adventures from Tales from the Outer Planes and Wildspace and I will also be using Castle Ravenloft, Cult of the Reptile Gods, Isle of Dread, The Dragon Sorcerer (cannon says no spelljammers cause its a closed sphere.. pfft cannon), and many other loved 1st and 2nd edition modules to end with an epic battle between Meph, the pcs and Corelleon themselves!
Done right this game should have humor (think Vagabond), High Challenging Adventure, Space Travel, Dungeon Crawling, Politics, Horror (Ravenloft), Danger (Darksun - Probably end up having to win their freedom as gladiators), Dragonriding (Dragonlance), Sinbad type (Al Qadim), and Planehopping.
Might even include dinosaurs as a cargo aboard a spelljammer which are bound for Chult.
Campaign Modules (In Order)
Tales from the Outer Planes - A Good Deed Well Rewarded, Castle at the End of Time, The Voyage of the Nerid, Wildspace, Ravenloft, scourge of the slavelords, my own darksun module featuring city by the silt sea and dragon kings, Isle of Dread, Genies Curse, Sword of Deceit, Tree of Life, Vault of the Drow, City of Spiders, Arcane Shadows, Against the Giants, Tomb of Horrors, Labyrith of Madness, Nightmare Keep and Earthshaker to final with the pre mentioned battle.