The Factions are a huge part of the Planescape setting, but fitting them into the 5e rules is more difficult.
One option we can go for is renown, as detailed on p.23 Of the Dungeon Master’s Guide.
The examples for renown focus on the group’s available in the Forgotten Realms, and there’s only five of those. Were trying to scale up to… Fifteen? Sixteen? Along with possibly giving NPCs a similar option for the Sects of the Outlands.
We can ignore ranks having different names for different Factions – we’ll refer back to the Planescape setting books, so effectively we have Namers, Factotums, Factors and Factols. The latter remain as an NPC (especially if we want Faction War to still be an option. Otherwise…)
|DMG Renown||Title (Harper)||Our Renown||Planescape Title|
Since the books say that the majority of a Faction is filled with Namers, and generally the higher ranks involve rituals and stuff, I think making it harder to get there should be important.
I started with a renown of 3 for Namer so that it takes a bit of work to join a Faction. That way you can do a couple of jobs for various and gain a reward, or even do some jobs for other factions without too much difficulty.
At 10, we give a title based on the Faction – for instance, in the Doomguard this would be the title of Doomguard.
So what should the perks be?
I think Namer is going to have NPC attitude perks – friendly instead of indifferent. Although you also have to deal with enemy Factions moving from indifferent to hostile.
Then each of the different Factions needs its own perk of some kind.
We’ll be keeping the restrictions since they’re mostly based on alignment. It fits with the setting.
We’ll also need some kind of infamy score, if someone were to jump ship to another Faction. We’ll look at that later though.