a)The characters had to be entirely original:
Skall, Neutral Evil Factol of Dustmen and as we know, a Lich, makes a bet with his buddy Silver Dragon, with whom they love to argue about metaphysical structure of the multiverse. Skall wants to prove that struggling with the pain of this life is meaningless and the only salvation is to shed all emotional connections and passions and enter the True Death. Silver Dragon thinks that through confronting person’s fears and past, also with the support of others, an individual can deal with their pain and heal. So Skall gathers the party of adventurers and sends them on quests with formal tasks, but with actual purpose of making them face their past “demons”. Party:
Bleaker Aasimar, Chaotic Good, Monk/Bard, was born and raised by a small cult as a prophet and “holy warrior”, but then discovered that his angelic father was deceived and drugged in order to conceive him.
Fated half-elf half-orc, Chaotic Evil, Ranger/Barbarian, pirate, marauder, mercenary, bandit, betrayed and betraying again and again.
Tieflings, brother and sister, mother was a whore in the brothel on Lower Planes, one was fathered by baatezu, the other one by tannari, Mercykiller and Chaosmen.
Doomguard Chaotic Neutral thief girl, looks and presents herself as a fire genasi, but is actually a human who grew up in the ever-shifting blue fire maze of Tower of Fire on the Mount Celestia and was changed by it.
And someone else, the second Lawful I guess, lol, I don’t have an idea for now, Lawfuls are always hard for me.
b)The characters all had to be preexisting characters from any Dungeons and Dragons setting.
1. Parted Veil’s go go education team: Athar illusionist gnome Kesto Brighteyes, Lawful-Good Sir Cleve the undead bodak and me (don’t you dare to tell me that I’m not a planescape character), also adorable rogue modron and maybe someone else run the coolest bookshop in Sigil, saving banned books and educating people. Planeswalker Candrian Illborne is a reoccurring character, who helps us to travel planes in order to find the rarest books and overthrow tyrannical regimes ^___^
2. Actually I’d rewrite the whole Kelemvor - Mystra (Midnight)- Cyric ascending to godhood affair, because I think it was handled ungracefully, and I’d wish to add more psychology and nuances.
c)The characters all had to be preexisting characters from a non Dnd setting displaced into the Planscape universe
Idk, I can’t think of basing *my* dream series around someone else’s characters? Like, I looove AUs, but in a sense of thinking up the context that could create the character with the same psychology and adding nuances. But dedicate the whole series to the retelling same character arcs in the new setting? It just doesn’t float my boat, man.