I think it'd be interesting, if a bit cliche, if Skeletor was separated back into Demo-Man and Keldor, and it was the big scary demon dude that was scared of the other. Keldor was the one in the driver's seat with Demo-Man locked in the trunk this whole time. Other than a toy bio, which I'm having trouble finding, we don't really get much on DM do we? For all we know he could see the situation the way Tuvar and Badra see theirs. Hell, have DM be the one of the pair that turns good, gets redeemed, and becomes a Master. Last thing he wants is to become Hordak's prisoner or be merged with someone again.
I don't think that's cliche. Its a trope, sure. But not all tropes are cliche.
But I also just don't see it. Keldor being the evil half, or "more evil than Demo-man" just doesn't track with what's shown of Keldor in canon.
In canon, when Keldor is banished from Eternos, his throughs are not of murder and revenge. His thoughts are of justice and unity.
One language, one people, one planet.
Those aren't the thoughts of an evil man.
And then as he's traveling the globe searching for supporters, most of them join Keldor because they're outcasts like himself. Or are branded as criminals like Keldor was falsely banished as a criminal.
And then during his final death, after He-Man does banish Demo-man from him. He reverts back to being Keldor, but the version of Keldor before Hordak "healed" him. All his injuries from his battle with Randor, and his acid burn return and Keldor lay dying at He-Man's feet.
His final words are not threats from the afterlife, or vows of everlasting revenge. His last words are a plea for peace and that Adam be a good and just king to all people of Eternia, even the non-human people.
These are not the desires of an evil man.
I understand the desire to make Keldor the evil half and Demo-man the one who was just along for the ride. It is an interesting reversal of common interpretations.
But I just don't see it supported in canon. If you do write such a fic (and if you like the idea, you should write it, I encourage you!) just be sure to tag it "AU" and "OOC".