John Byrne killed Master Pandemonium. Well, he killed off the interesting bits of the character, at any rate. Y'see, that pentagram shaped hole in his gut is where former movie mogul Martin Preston's soul used to be, before he made a deal with Mephisto. Mephisto also replaced Preston's missing limbs with demons. Angry ones. This is what's known in Hell as a 'perk'.
Now, in his original appearances, Pandemonium was consumed by his quest for the five shards of his soul, which Mephisto had scattered to the four corners of the Earth because he's a dick. Mephisto, not Preston. Preston was, by all accounts, a nice enough sleazy producer/actor before running afoul of a moonlit canyon road and the devil, in that order. Preston's whole modus operandi is his search for the bits of his soul, a search which is slowly changing him from a normal man into a monster of the first order.
Then comes Byrne. Preston goes from tortured soul-hunter to demonic deus ex machina. Not at first. At first, it's pretty much par for course for Pandemonium. Then it's revealed that he's nothing more than a homunculus-a servitor designed to retrieve the missing portions of Mephisto's own soul (which happened to be the Scarlet Witch's kids)! Mephisto lied about everything! There never was a Martin Preston!
After that, Pandemonium suffers a fate worse than death-he becomes generic. An ugly demon-sort of guy (at least until he pops up in Young Avengers) who does demon-y things, until his most recent appearances where he appears to be suffering from pangs of guilt ala his early days. Hence, 'John Byrne killed Master Pandemonium'. Byrne eliminated the hook that made the character interesting in order to tell his story. So, to make MP viable again, you've got to give him his hook back. How, you may ask? Simple.
He lied about lying, in fact. All part of the game. He manipulated and used Preston in order to achieve his own goals. Preston's soul bits are still out there (all five of 'em), inhabiting the bodies of five unaware innocents (or not). Too, they move. Unless he catches them at the right moment, they switch hosts, forcing him to start his search all over again.
And, having tasted the torments that await him should he fail, Master Pandemonium is now more determined than ever to reclaim what's his, regardless of the cost. Or is he? Preston is determined and slightly crazy, but he's not a psychopath. Desperate, yes. Sadistic, no.
Which means that his hunt for the soul fragments might not be as cut and dried as he hopes. What if there's another fragment lodged in a child somewhere? Can he take it-thus killing the child-without Mephisto's influence goading him as before? And what if Mephisto has put stumbling blocks in his path-demonic adversaries that seek to engage Pandemonium in battle so that he misses his chance at a fragment?
Imagine that-demon versus demon in downtown suburbia, with the hapless Pandemonium forced to act as a protector of innocents in order to have any hope of claiming his soul-piece.
Of course, there would be others after him as well. AIM, for one, looking to exploit the fact that Preston is essentially a dimensional doorway made flesh. And, any hero in the vicinity is going to get involved, right?
It puts a unique spin on the character. A villain who's a ticking time-bomb of destructive potential. How many interruptions to his quest can he stand? How long until his humanity gives out beneath the weight of his frustration? What happens when he finds a fragment inside one of Earth's Mightiest Heroes? What if Mephisto put one in Aunt May after Spider-Man made his deal? Amadeus Cho? One of the few remaining mutants?
Hell, what if one gets stuck in Deadpool?
Lots of possibilities there.