Gearing up for running a 5e Planescape game, and I've begun working on converting the setting...
[sblock=Musings]I decided that a straight conversion was a bit limited, so I'm taking artistic license. All the familiar stuff will be there, but I am tweaking the factions with an eye toward playability. I am also thinking of accepting the established continuity (yes, even Faction War), but keeping the focus off the past and instead focus on the wonders of the planes, on the PCs, and what lies thru the next portal (rather than an obsession with meta-plot or a sys-admin view of Planescape).
The factions will be in Sigil, there will be a lot of them (more than 15), but they won't be the top-dogs there, rather they'll be more like secret societies; for example the Fraternity of Order will have members who've infiltrated (or openly serve) the City Courts, but the City Courts aren't "theirs" exclusively. So you might have judges from competing factions conflicting with each other, judges conflicting with dabus judges, accused citizens claiming their judge is a Guvner to discredit him, and so on. Turns out the Lady of Pain's decree in Faction War meant the factions couldn't have their headquarters in Sigil nor control it's government, not that they were totally outlawed. Still they operate more in the shadows than out in the open because of this, so the kriegstanz is on in a major way, and the entire planes are their battleground.
Here are some of my brainstorming notes...
Divide races into "classic" and "strange". Classic races would include aasimar, bariaur, genasi, githyanki, githzerai, rogue modron, nathri, and tiefling, along with planar half-elves and humans. Strange races would include bladeling, foo dog, reave, and tso from Monstrous Compendiums, along with new races I'll design: Fused being, godling, mimir (sentient), petitioner (exceptional), planar shadow, and Yggdrasil spirit. I think that covers all the archetypes and allows for more weirdness/diversity that players enjoy and heightens the feel of it being Planescape.

I'm really excited to share my write-ups for those new races...there's some delightfully strange "abstract made tangible" themes I'm playing with there!
Factions designed as backgrounds with an extra feature (2 instead of 1) that also includes a hindrance. In addition, each faction will have an associated feat inspired by Factol's Manifesto. Sometimes I'll deviate from the cannon abilities however if they don't make sense or if there's a better option. Thinking that the Basic D&D/PHB backgrounds will be for Clueless PCs. Will be changing a few factions, notably Fated to Fateless (who deny predestination, cheat the Fates, and make their own way), a more sympathetic Harmonium, a Free League that really stands for the principles of freedom/free market,
Reinterpreting Inspiration as Belief, and awarding it for adhering to an Ideal despite great personal cost or suffering. Also providing more uses for Inspiration, such as attuning to a planar touchstone, eschewing spell components, gaining an intuitive clue, or (most importantly) re-shaping the planes am contemplating some uses of re-shaping the planes requiring multiple applications of Inspiration, whether over time or thru multiple PCs working in concert. For example, Inspiration (x5) to shift a gatetown on the brink across the border from or into the Outlands.
In addition to faction feats, adding 2 new feats: Portal Feel & Planar Sense, which will be adaptations of the proficiencies in Planewalker's Handbook.
Magic item conversions from Factol's Manifesto, Planewalker's Handbook, and that old Dragonarticle for 3.5e.
New spells selected from Planewalker's Handbook, with an eye toward cutting down their number to just the essentially unique ones.
That's it on the player side... Hah!

Quite a lot of converting/updating actually! Once I've got the basics hammered out, I'll post it to the boards.[/sblock]