1-PC campaigns can be a lot of fun, but it's tough to publish an adventure module for them. A lot depends upon the individual PC. If they play a fighter, for instance, you need to be sure (especially at 1st level) that they won't need much -if any- magic to succeed. Not a Rogue? Better not be many traps, etc.
I run a solo game for my wife and it is a lot of fun. We play as part of a group as well, but once in a while she pulls out her solo character and we play a short session. I use Basic D&D rules for the game, but there's no reason not to use 3.Xe if you prefer.
I tend to de-emphasize combat and crawls and use more quests and mysteries/puzzles. In her current adventure, her dwarf is trying to rescue a boy who was kidnapped by goblins. She had a combat with a giant spider (about 2' across) as well as killing 2 of the 3 goblins and finding out where the boy was from the 3rd. They sold him to an ogre in the swamp.
She still has to find a way get the boy from the ogre, but I've planted a few clues as to non-combat solutions. Also, there's a friendly blind cleric in the village who gave her a healing potion to help.
Just a thought, but you also might check out some of the MANY solo adventures written back in the 80's & 90's for the Tunnels & Trolls RPG. I think some are online, too. It shouldn't be too hard to convert some of them or just lift the basic ideas.