So I've decided to get down a campaign/adventure/story thing for my one player game... http://www.rpglaboratory.com/james_simon_faust/one_player_game_ideas
I've written down a few character type ideas and thought up a lot more that may or may not get into this particular adventure.
The setting is a world not that unlike our own save for magic, robots and monsters. A world ruled by a king known as the Agkmari. The adventure focuses around the player, a young student who has one dream in life... TO TAKE OVER THE WORLD! so with only a few points and a bat our young hero and his/her friends (guided by a wisecracking narrator!) set off to beat down the King and rule the world!
as the story progresses you will be joined by a number of allies that will have effects on story events based on their character type. races/classes seemed to specific. character type seemed more fitting for the mechanics of the game. Powerful Yetis, agile cat-girls, dark magic using devils and infamous bad-asses are just some of the characters ready to drop what ever they were doing to join you in the battle for world conquest. having members of different character types in your party can trigger different story events or give you an advantage (or disadvantage) fighting certain powerful enemies.
The game is played in chapters that must be cleared by the player finishing all the events contained within, earning a certain number of points or laying the smack down on a major enemy. I'm not 100% how many chapters the game will have, all i have right now is a beginning and 2 or so alternate endings. Also the mechanics have been altered a bit from the ones in the vague rules in the link but they still work to get an idea of how the game is played.
and to end this i ask a questing... should party members be swapped out at a library, since theres really no taverns or inns and i kinda like the idea of a library (thats where I'm posting this from actually)

Infinte options?
Well, it depends on where the library is, how many potential NPCs there are, and how they affect the story. What happens if two or more NPCS apply to the situation, or if you have conflicting personalties (a demon and a priest for example)?
In a conventional RPG, the GM could just rulings on the fly about this, but as a single person adventure, you need to program each deviation... which could be a lot of work.
My suggestion would be that the chance to alter party composition comes at the beginning of each chapter. This helps enforce a little tactical thinking and caution, since you can't backtrack to pick up a replacement (right?) It also means the chapter only needs to account for NPCs you want available for that chapter, rather than any met so far.
I'd also probably argue that the easier approach would be for each NPC to be part of a category - ie the dark, the athletic, warrior - so that the responses can be "if you have this type of person in your party, then..." rather than a response for each individual. Once again, its the potential for dozens of branches to form off the line much like ripples in the dirac sea or certain theories about quantum physics and infinite universes. (Time travel... paradox in a can)
Story wise, meeting in a library seems like a good choice.
There is a fine line between hobby and obsession. I seem to have lost sight of it some time ago.