The following is a short list of the concepts to be presented in this Post-Sigil multiverse. I'm considering redoing the thing completely, making it more of an homage to Planescape rather than a literal Planescape successor.
The Fall happened millenia ago, with the survivors regrouping along common philosophies. Though each society has respected members and ancient sages, there are no recognized leaders. List below are the most common societies, though many others exist.
With the death of the multiverse around the corner, possibly with a body's lifetime, why not succumb to what little life offers? You can create the world around you (which, sadly, is in ruins), so why not build it to please you?
Entropy and death have claimed the multiverse; the goal in this mortal life is to embrace the inevitable and prepare for the end. Some Entropes claim that life and death are cyclic, and the worlds are close to rebirth; all the more reason to rise to great power and fame. If the Entropes lives to see the resurrection, they may be born again as gods.
A Kaologos observes that law and chaos are bound by observable rules. Within law there are imperfections and, alternately, patterns can be found in seething chaos. A strange combination of mysticism, scholarly study and physical effort. Existing within the multiverse should happen at both an intellectual and an intuitive level; the true Kaologos cannot separate intuition from reason.
Named after an ancient hero, the Odysseans see this mortal realm as a set of trials to be overcome. Some believe that a great reward lies at the end of life, others struggle for fear of what comes after death. In addition to this common philosophy, Odysseans share a certain martial attitude, and often act as hired muscle or voluntary militia. Those with a penchant for roaming often lead bands into terrible danger, believing that the greater the trials faced in life, the greater the final reward in death.
In the face of the desolation and inevitable doom before them, Ring Givers echo the classic adventurers from Norse legends, acting through nobility and courage and selflessness. They also recognize the value of mercy and compassion, knowing that helping others also helps one's self.
The Fall led to a great loss of belief, and with this loss the boundaries and laws governing the planes themselves began to disintegrate. Now the planes are a vast jumble of shadow realms and epic landscapes bleeding one into the other. It is possible to traverse the Entangled Planes both physically and spiritually, along common and uncommon routes.
If one were to map the Entangled Planes, it would be difficult but plausible. They are often arranged in a ring: Eanna, Ardor, the Wilds, Lazar, Abaddon, Dystopia.
A great blasted wasteland and what remains of the various hells. Ruins of vast realms, from icy fortresses to firey iron cities, crumble and shelter what few infernal occupants remain. Self-serving mortals tend to be drawn to Abaddon for many reasons, from lording over demon holds to conscripting terrible armies. Occasionally great demonic warbands march out of Abaddon to lay waste to the Entangled Planes; sometimes resulting in entire realms merging with Abaddon itself.
This is a passionate plane, most connected to the mortals. A chaotic place, run by a mortal's raw emotions and the most human of gods. Some say many of Ardor's gods have abandonded their immortal posts and wander the planes as epic yet mortal heroes, eager to die as a hero rather than the shadow of belief.
Dystopia is both order and oppression, one where survivors have tried to prop up the established bureaucracies. Though this monumental task is hardly up to the fickle and short-lived mortals. It can be a benevolent machine: There are those who revel in following the rules and those who delight in breaking them. This is the only great plane which exists as vast architecture instead of a more natural landscape.
The heavens have merged into a plane both kind and stern, known as Eanna. Unfortunately, the merciful gods are dying along with the horrid ones. Eanna is the most peaceful of the Entangled Planes, but its paradises exist only as shadows.
A horribly diseased plane, filled with the madness of mind and body. Beneath the wretched surface this plane is riddled with catacombs and natural caverns. Lazar is often used as a prison, though many mortals here stay through self-loathing.
Anywhere nature has overrun the land is merging with or wholly part of the Wilds. Animal nature prevails here; some say Ardor and the Wilds are slowly combining, if not already on and the same.
Here's a quick system that is highly player-driven. This should allow for
Obviously, this setting is inspired by Planescape. What follows is the list of those inspirations, plus any others. It's frustrating when a game or setting doesn't list its inspirations (D&D itself borrows from Jack Vance's Dying Earth, Tolkien's books, Fritz Lieber's Fafhrd and the Grey Mouser, and Moorcock's Eternal Champions among others).