He was the main antagonist in a High Noon story already, so we've got the backstory ready to go. Yes, he died, but that historically has not stopped Karthus ever.
Plus it'd give us yet another scary faction in the High Noon line. He actually suppressed Lucian's demon for a bit before the whole "shot in the head at point blank range" business, so he's definitely got some pretty powerful magic. Karthus' book had a key on it and his magic was spooky teal instead of infernal orange, so whatever he's got going on it's not devilish in nature. It also wasn't angelic gold, but I'm _pretty sure_ we don't need the color cue to tell he's not with that faction.
As for his current motives, you can't go wrong with "I'm looking for the jerk who shot me in the head and burned down my goth hippie commune." That's classic right there. Well, the first part is at least.
I doubt he's coming out any time soon given that we just got three High Noon skins. But I think there's a strong case for him to show up sooner or later.