If you've ever encountered the TV series Doctor Who, you'd know that his adventuring party -- sorry -- companions don't always get out alive. Which makes the Doctor, or something like him, perfect as a campaign premise.
The Doctor is very much a deus ex machina, armed with two other deus ex machinas. Not only is he 'the last TimeLord' with powers and abilities sometimes limited, sometimes vastly beyond those of mortal ken -- he also has a T.A.R.D.I.S. (a sentient, timetraveling, multidimensional device that can shapeshift but is frozen into the form of a police call box), and a sonic screwdriver (you read that right). He is brilliant, fallible, unpredictable, and capable of terrifying reversals of fortune -- which his opponents have found out, much to their dismay.
From a campaign perspective, he provides some much needed continuity (if not direction) for games where a Total Party Kill can occur from time to time. He also allows characters to gracefully retire (with an option to return in some future incarnation), and allows for new characters to join the story.
If we were to use The Doctor as a template, perhaps for a Mystara campaign, we'd want to retain certain key elements to the nature of the character:
- brilliant, arrogant, always knows best, fallible, dangerous, merciful, feared by many, loved by a few, with an annoying tendency of not always wanting to explain what his plans are until they've succeeded or gone horribly wrong;
- has lived a long time; has forgotten more than you'll ever know; has encountered more things than you ever will;
- has a device that can travel in time and space, but isn't the smoothest or most reliable of rides (and the Neil Gaiman-penned episode gives a suggestion as to why);
- a keeper of secrets -- dangerous ones, many best left buried;
- assumes, sometimes incorrectly, that all his companions are as capable as he assesses them to be;
- likes to travel;
- plays favorites;
- has a different sense of morality from the rest of his kind;
- has a fluid personality and skill set from regeneration to regeneration;
- occasionally teams up with past and future incarnations of himself.
How would I introduce him? Well, I'd start off with a riff on either X2 -- Castle Amber (Chateau d'Amberville) or the first adventure in the Wrath of the Immortals campaign. An imprisoned man with strange powers that seem to confound and defy known principles of nature and magic -- he'd either tag along or constantly 'bump into' the members of the party that he likes, gains their trust, sends them off on missions, sometimes accompanies them...
... actually, Wrath of the Immortals is an excellent jumping off point, since there is a part of the ending wherein beings more powerful than the Immortals appear and set metaphysical things right (not necessarily all the countries decimated, lives lost, etc.) which is a nice echo to Gallifrey and the TimeLords and the Time War and the Daleks and Rassilon and the Hand of Omega and the Nightmare Child...
A campaign like this would mean that the PCs are major players of a sort. They can change the fate of the entire timeline, regardless of character class or level, by being in the right place at the right time. By helping steal a weapon or destroy an artifact, by helping a dynasty rise to power or by helping avert a catastrophe, by sending key individuals down the right paths or by becoming beings of immense power and influence themselves.
They could go anywhere -- to countries present and past, to the Hollow World or to one of the moons, to hidden dungeons in the depths of the earth or to floating fortresses in the skies, and to all the known and unknown dimensions.
Furthermore, adventure can find them. The Doctor has had his enemies and friends leave messages and traps and gifts for him in the strangest of places -- why not our analogue in Mystara?
Another thing: if there is indeed a time war being waged between our Doctor and some other Uber-Immortal, it allows for certain things to be changed from the last time you visited either Alfheim or Darokin or Alphatia or the Empire of the Ghouls or the Shires of Yluaram. What do you mean? You've never heard of the Shires of Yluaram?