The Miraculous Nobilis Engine
Nobilis is an amazing RPG with groundbreaking diceless game mechanics. Its author, Jenna Moran, later expanded those mechanics into the Chuubo's Miraculous Wish-Granting Engine — adding a bunch of new miraculous attributes/arcs, improved action resolution, and (most important) a functional XP advancement system.
It shouldn't be too difficult to bring those updates back into the world of Nobilis.
The Mission Statement
The setting is 100% drawn from Nobilis, while the mechanics are drawn from Chuubo's unless explicitly stated otherwise.
Anatomy of a Noble
A Noble is built as a Chuubo's character, with 8 points in Skills, 8 Will, 5 MP (a single pool, not four different ones), and 8 Perk Slots (most of which start empty). There are no longer "Gifts"; those have been subsumed into Perks and the way that Miraculous Arcs are more fleshed out than Miraculous Attributes were.
Each Noble starts with 9 levels in Attributes, all of which range from 0-5. These are still known as Aspect, Domain, Persona, and Treasure, but these have changed significantly from Nobilis and some have "wiggle room" for expanded interpretation.
By default, this uses The Ace. However, if the Noble is more magical than talented, they may substitute Chosen One instead.
(Should Reality Syndrome be on the table? I don't think so, but am considering it.)
This uses Spiritual. There's no room to adjust this one, as it's key to the function of Nobles.
By default, this uses Star Quality. However, if the Imperator has given the Noble a non-Estate role to focus on, they may substitute Become Somebody. Or if the Noble wasn't originally mortal and it makes sense, they may substitute Allegory.
By default, this uses Sentimental. However, if the Noble is tied very closely to the Realm, they may substitute A Keeper of Gardens.
(Should Impresario be on the table? Dunno.)