Not so much a full system, but part of one...
The last Campaign Setting for AD&D 2nd Edition was 'Birthright'. For those not familiar with the setting, it was Arthurian mythos with some extra cultures thrown in, plus a smidge of Highlander (your character could have the blood of a God in them and gain more power by killing other Blooded characters by stabbing them through the heart with the setting's equivalent of Mithril).
Part of the big focus of the Campaign was the whole Arthurian idea of 'The Land and The Lord Are One'; by having a Blooded character, you could be tied to the land in various nifty ways (Wizards got awesome Battle Magic that they had to use ley lines/sources to draw upon in rituals in order to cast). Each of the Kingdoms/Nations in the campaign world were divided up into smaller areas called Realms. Each Realm had numerous Domains, representing different kinds of power bases in the Realm: Law, Faith, Trade, and Magic, IIRC. Your Blooded character could develop ties to any of these domains to start a power base. For example, a Blooded Thief character could start out with 1 of a Realm's Law Domain to represent bribed judges and guards, a Blooded Priest could start with 1 of a Realm's Faith Domain to represent the Temple and lay folk they influence, and the Blooded Wizard goes for the Realm's Magic Domain, of course.
During the course of the campaign, there's a reasonably simple mechanism to simulate running Domains, including trying to grow your own power base, take Domains away from NPC's, espionage, direct warfare, etc. It added a really great, simple political acquisition & conflict mechanism to the standard AD&D game that could be tailored to a specific group's campaign goals, whether it was 'reclaim a character's ancestral holdings that were stolen a generation ago' or 'install a PC (or NPC) as the new Emperor'.
I'd love to see this tried or replicated with a fantasy game to focus on the socio-political maneuvering that can happen in game. Hell, I'd love for there to be a Birthright game in general. The Birthright online community has an 'Unofficial' 3.0/3.5 conversion pdf, and are working on 5e, or may be done with it.