A sole product I'd love to see.

[Maraxle]

I find solo adventures exceptionally easy to run. I just make the player provide me a very detailed background, motivations, etc, and run with it. I generally create an NPC sidekick for the main character, unless there's something in the character's story that would prevent me from doing so (like that he's a loner and hates people). Other than that, just avoid spells that can incapacitate or kill the character too easily (like don't Hold the player in a pit full of crocodiles).

I find that lower magic campaigns are more conducive to this, as are beefier, fighter type characters. The most successful character I've seen in a solo game was a swashbuckler-style fighter, with high dexterity and good strength, doing lots of daring flips and such. At one point he was incapacitated, so I had the bad guy beat him and throw him into a cell. A whole new adventure came out of trying to get out of the cell, and eventually out of the building he was being held in.

 

[Sir Edgar]

If done properly, these could be good intros to the game for people who are not familiar with D&D. They could be kind of like "D&D lite" in the style of "Choose Your Own Adventure".

For example, it could have a pre-made PC with all their stats, equipment, and personality listed in an easy-to-understand way. The adventure should be simple and yet fun. Complex combat scenes would be avoided. Puzzles and problem-solving should be an essential part of the game, however.

On the other hand, a really complicated challenging adventure for experienced players who want a solo game can be good, too.