Two other "Imperial civil war" type conflicts I've found in fanfiction. Both from AU stories, naturally.
Darquecyder 6's "A Simple Decision." Set in one of those Leia on Tatooine, Luke on Alderaan universes. Kind of amusing, really- Leia is of course mechanically inclined, but is also Biggs' lover and for some reason Obi-Wan is present in her life as a distant weird uncle. Luke is as passionate to the Rebel cause as Leia was, but instead of a master diplomat he's a teras kasi-toting bad-ass trained by
this guy. I like this story for a lot of its eccentric touches- Leia's droid is R5-D4 (who got shafted in the film) even though she buys the other two droids, and she's accompanied by a jawa shaman and an AWOL stormtrooper when she leaves Tatooine with Ben. I suppose it's a bit EU wanky in the sense that it incorporates a lot of the stuff that we know "in retrospect", so a lot of the expanded details are included, which makes it rather less nice and simple than the classic films. For one thing, there is no Battle of Hoth- the Rebellion relocates to
Rexus Prime of all places, where Luke discovers some KOTOR artifacts. And both Kyle Katarn
and Jerec are present during the Battle of Yavin.
The Imperial schism occurs between its alt-ANH and ESB. Zaarin's behind it, but he basically links up with
a lot of EU and original characters, not to mention people like Moff Trachta.
The other one is one Darth Yoshi's Infinites stories (
"The Apprentice",
"The Warrior",
"The Master",
"I, Sith", all can be found
here. They begin in an alternate TPM, where the Jedi Council refuses to train Anakin and he's taken in by Dooku instead. I kind of skimmed "The Apprentice", because I wasn't too interested in it (it was written in 2002-3), so it seemed a bit dated, but it ended with Amidala becoming a fake Force-user (with powers artificially boosted by the Kaiburr Crystal and her connection to Anakin), basicaly a weak Lady of the Sith. Luke is raised by her as an Emperor's Hand, Leia is still Bail's foster daughter, and there's a lot of stuff going on with Corellia (which is naturally a very autonomy-minded planet that's really disloyal to about everybody). In any case, in this version, Tarkin believes that Palpatine has sold out his own values, and basically takes the Death Star over Corellia and rallies a section of the Imperial Fleet to defect with him. However, only a few admirals do, including his successor Daala and the EU-character
General Loor, and the "Tarkin Confederacy" never really gets a lot of support nor success, though they
do have the Death Star. I'd guess it would be akin to Himmler and other hardliner Nazis trying to secretly oust Hitler because they view him as old and weakened, except in this case Hitler can Force-choke you from light-years away. Which makes the story a bit hard to swallow.
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Personally, the sort of alternate devolution of the Empire I'd make is influenced by some of these AU's, as well as ideas from threads like the aformentioned one that Surlethe linked to.. The underlying idea I guess is kind of like an alternate history where the Third Reich doesn't fall, but is caught up with internecine conflict following Hitler's decline. And so it ends up with ironic scenarios like that of
The Man in the High Castle, where ironically you're more inclined to support the horrific Heydrich than the rather more sane Goering. I'd split the Imperials into three vaguely drawn camps of different sizes (the last is the least) following Endor- you'd have one populated by New Order hardliners who believe in preserving Palpatine's ideals and policies (and probably would fall over themselves in rushing to worship him if he- or a canny enough impostor- returned), one populated by Tarkin technocrat state terrorizers, and one populated by his scattered Dark Jedi apprentices, inquisitors, and dark prophets trying to create a new Sith Empire (though seeing as how all of them are relatively weak, scattered, and caught up with in-fighting, all they can really do is subvert state appratus to their power, gather cultist followers, and try to summon one of those Force ghosts to help them). And in the background, of course, is a lot of rival moffs and admirals playing warlord. It's a bit like the Russian/Chinese Civil War- there's a few major (ideologically-aligned) factions, but there's a lot of local warlords as well.
I'd try to model this conflict based on the French Revolution, possibly, with differing factions gaining power but with limited control of the state and never for a long time. (It's also a bit like the SW MUSH history, I guess).