Thinking of starting a planescape campaign (Input needed)

Hmm, unnatural or improbable aging would suggest to me something like a ghost attack was involved (or maybe whatever magic was keeping the old man alive was dispelled).

Anyway, this setup gives me three questions:

1) How'd the wizard get this debt in the first place?

2) Who did the wizard owe the debt to?

3) Who wants to get the town under glass? (Palomides has a neat few ideas there of potential buyers).

1 and 2 will be important for your PCs to discover the context of the wizard's death going backwards, while the third one will be important setting the context going forward.

The Govners could easily find a legal reason to confiscate someone's property after their death. Perhaps it is even legitimate - the man was involved in some kind of litigation at the time of his demise. Perhaps he was being sued by someone else who fealt they had claim to the city in the globe - a supposed escaped survivor, the representative of a minor diety or a demon lord who had purchased the land in trade from its rightful ruler. Perhaps all his do-gooding simply cost the wizard more than he could afford and he was taking out loans to finance his good deeds.

Perhaps the person who takes possession of the globe actually gains a godlike power over the land trapped inside. He can alter the world within at will and is whorshipped like a deity. If this whorship became too powerful he might have been killed by the Lady of Pain herself as he became too "divine" ... and divine creatures are not welcome in Sigil.

After the Wizard's death the pcs accidentally "inherit" the globe - perhaps as the effect of a divination spell gone awry as they attempt to scry what happened to the Wizard. (the spell goes haywire and the magical currents bind everyone in the room to the globe simultaneously) Suddenly the party is in an awkward position of being a new set of "gods" to this entrapped world.

This would branch out into a vast series of adventures while they influence their world from without ... when a plague breaks out inside these "gods" have to discover some way to cure it in their own world and then use it upon the globe ... when demonic entities start to breach it they have to choose and outfit "heroes" within the globe world to fight the evil. Basically you've got a way to let the pcs play "gods" without really being gods. And then there is the issue of how each of these new pseudogods manifests itself in the other world and how their personalities - and thus their followers - clash with one another.

Then, of course, there is a balancing act here - in that if their whorship becomes too prominent in that world they will have to flee Sigil before the Lady makes examples of them ... not to mention as other creatures discovered what they had at their fingertips there would be those trying to get their own hands on it.

Since I really like this idea (i.e. I plan to lift several elements from it), I just wanted to pose this follow-up question:

If the campaign focuses on the mystery of what happened to the wizard (and maybe what happened to the city - if one uses the idea that it is now missing); what, aside from the highly fantastical nature, makes this a Planescape campaign?
My point is that a lot of this could play out in any campaign setting.
My intial suggestions (which seem to be going against the flow that most are pursuing) were to place the PCs as fish-out-of-water where they were bumbling around the planes trying to figure out what was going on.

If we keep with the mystery angle of "what happened to the wizard", what is the motivation for the PCs to go plane-hopping?
Would the mystery of to whom he owed the debts lead them to one plane (and if so, what are some suggestions)?
Would the mystery of who killed him lead them to another plane?
Would the mystery of where the bauble/city is now (if one has it gone missing) lead to another?
And would the threats of other groups wanting Kandor lead them to (or away from) other planes?

Yes, I realize the answer all these questions would be "Sure, if you want it to" but I am curious about some of the concrete ideas percolating in the various minds out there. Whom do people see as the major agents in this drama? What forces are at work? How is all this tied to Sigil and/or the planes?

I'm starting to think I'm a thread-killer. Nonetheless, here is one possible iteration:

On one world in the Prime, over time most of the mages turned to the power of demonology [name being used generically for magic powered by any sort of fiend] to increase their magical power. This situation grew at exponential rates since the magic users needed to keep themselves ahead of their rivals, who were doing the same. Eventually, it turned into a form of arms race in which the fiendish races gained more and more of a foothold on this world. Eventually, that world became a major battlefield in the Blood War.
There was a brilliant mage from this world who had left ages ago before the magical power of fiends were harnessed. Upon returning, he was aghast at what had occurred. While it was too late to send back the fiends or to undo the damage he had done, the wizard found one city remained that was a bastion of purity, At-Ijal [this city had a more contemporary culture with elements drawn from ancient Egypt]. This city had turned away from the practice of summoning fiends and the theocracy that ruled here put in various magical wards to prevent such magics. This stance drove out those seduced by the Dark Arts and attracted those who wanted nothing to do with fiends. Fortunately for this city, the opposing forces of the fiends were more focused on each other than they were on this one lone holdout.
The mage realized that the luck the city had experienced wouldn’t last forever. So he devised a powerful magical ritual that would allow him to shink the city into a small, easy-to-carry bauble that he could then carry to another plane or world where it could be restored. The wizard resigned the remainder of his home world to the fate it had brought upon itself [and it now lies as a burnt cinder.]
The wizard and his apprentice then started on a campaign of exploration through the Great Ring and beyond searching for the ideal setting to relocate At-Ijal. The wizard, being a perfectionist, always found some fault with the possible locations he found. As the people of the city were content and grateful, the wizard took his time. Ironically, rather than relocating the one he had, the wizard found additional cities in need of rescuing. These included:
*Bensonville – a city in the Outlands that was in danger of being crushed in the Great Modron March
*Chentalar – a floating city [Polynesian/Oceanic culture] that was about to sink under the forces of a monstrous typhoon
*Djordafur – a bariur city from Ysgard that was being beseiged by fire giants
etc. [I’m going with an alphabetical theme to keep things straight]
With the growing populaces under his roof, the wizard’s apprentice began to think about the implications and uses of having various civilizations under one’s thumb. If nothing else, the wizard (or the apprentice himself – if the wizard was removed from the picture) could collect a small tithe on each trading mission sent to the markets of Sigil from each of the respective bauble cities.
Because of this, the apprentice actually encouraged the mage to collect more and more cities (although the apprentice always couched the reasons in the most humanitarian of terms). Eventually, the acquisitions included the absorption of the PCs’ home of Kandor [sorry, I can’t resist the Brainiac reference]
The increasingly greedy apprentice was not powerful enough to cast the spell needed to shrink a city; but after a certain point he ceased to care. Enough cities had been collected that they would generate enough money in tithe so that the apprentice could live in idle luxury without needing to do any work. Shortly after reaching this decision, he drugged his mentor's evening meal and then killed him in his sleep.
At first the apprentice told the agents from the cities that the master was out searching for suitable locations where they could live. But before too long, the cities came to accept the new status quo and almost forgot about their original benefactor.
But the nature of greed is like an infection, soon the greedy apprentice and those outside forces that were curious how he maitained his wealth intiated a chain of events that would lead to the murder of the apprentice and to the mystery in which the PCs have been plunged.

I won’t elaborate on what events went down that resulted in the apprentice’s death. But for my tastes, I would definitely add a wrinkle that the powerful magic used to create small pocket realms that can be brought into Sigil will attract groups outside of just the simply greedy.