Well, Sphinx (http://sphinx.pocoo.org/) generates really good Python documentation. Perhaps if you were able to come up with something that also generated nice documentation, but was language agnostic, that might be cool.

I'm also a fan of the recently popular annotated source code style of documentation. Although I feel like this style of documentation takes more effort to use.

If your service just hosted API docs, perhaps you could just make a platform that allows people to choose their API documentation platform, such as Sphinx, and somehow make it easier to generate it and deploy it on your servers. This might be enough of a reason for people to use your service. I wouldn't pay for it, though, unless you had some compelling reasons why I should. GitHub makes their basic project hosting free and it provides much more functionality and utility than a API documentation service might.