JesseLowe/AlternateAbyssals
One that I've toyed with is that Abyssals are not corrupted Solars; they're a wholly different sort of Exalted, created by the gods to represent the dead and enforce the laws of reality in the Underworld. In short, they're the Underworld's version of the Solars. So what are the effects?
First off, Abyssals are not necessarily Evil with a capital E. Instead of campaigning for the end of existence, they can be the defenders of the Underworld against the threat of the Void and strive to maintain the balance of life and death. (Maybe they're the jailors of the Malfeans, striving to prevent the Yozis from freeing their undead kindred.)
Second, the Underworld need not be so terribly inimical, either, and can be reshaped as more of a part of the natural cycle; a stopping point in reincarnation for souls that are dead but not yet ready to reincarnate.
Third, it eliminates a major thread from the Exalted canon: The Malfeans are responsible for a lot of trouble in Creation, and, canonically, want to wipe it out. The Abyssals and Deathlords are their tools. Make the Abyssals "good guys" instead of "bad guys" and the Malfeans become more like the Yozis -- they have to work through cultists and demons (or the dead equivalent) instead of having nifty deathknight slaves. From the point of view of other Exalted campaigns, this is probably the biggest difference.
Fourth, what about the Deathlords? I'd favor setting them up as the Gods of the Underworld, part of the bureaucracy of Heaven (more or less).
Fifth, what happened to the Abyssals during the Usurpation?
Sixth, are there Underworld equivalents of Lunars, Sidereals, and Dragon-blooded? (I'd say no, if only because it'd be a lot of work.)
Mechanically, if going with this idea, you'd probably tweak the Abyssal Charms to be less "kill everything" and more, eh, death-friendly, I guess. Probably keep the dice-subtracting gimmick they have, though. Thematically, reinforcing the boundaries between life and death or weakening them as necessary, more stuff to do with ghosts and the Underworld... etc. (my inspiration suddenly runs thin)