Why does the abyss need to be mutually exclusive with a Chaotic Evil outer plane? You could have both.
It's not an exclusivity issue. It's that the Great Wheel's CE plane IS the Abyss, with all its layers and all that jazz. So you'd need something that could replace it.
My suggestion,
@Xeviat , is to take a leaf from Eberron and make the CE plane a realm of nightmare and madness. Perhaps the shard of evil was the "original" CE plane, but invaders from the Far Realm exploited Planescape-y logic to
dislodge it from the Great Wheel entirely. This sent it careening into the Elemental Chaos, warping its nature etc. Fortunately for reality, the denizens of the Far Realm who plotted this didn't really think their plan through, and now they're stuck in a limbo, where they've "gone native" too much to return to the Far Realm, but not enough to truly
live in the World Axis universe yet.
So they're plotting, scheming, tricking, coercing. Fostering Cthulhu cults convinced of the inevitable victory of the Far Realm over reality. Patronizing GOO warlocks who bring just a little bit
more of the Far Realm into the World every time they use magic. Rewarding wizards who summon things of madness and unreality. Infecting mortals with parasites of body and spirit. Maybe sending agents to try to awaken imprisoned Primordials or steal the lingering powers of dead gods. Etc.
This could even be leveraged for fun interplanar drama. Demons hate the madness-place beings more than even how they hate devils, because the former
usurped their place in the cosmos, by trickery, while the latter at least had the gonad gumption to
fight them for it. Devils were willing to put aside even the Blood War, temporarily, in order to keep the Far Realm contamination mostly contained, and naturally angels and other upper-planar beings helped out as well. The one time Good and Evil
all got along, when Unreality tried to take over.