The "AI API" is the dream application. Just imagine: you get to implement (and even discover) cutting-edge prediction algorithms, make them scale, and expose them via your protocol-of-choice. No frontends to write, no Joe Luser to support, just beautiful math and hardcore infrastructure engineering.

There are reasons this hasn't been done before. I don't think it's a viable startup idea. Think about the capital you would need to even get something like this off the ground. Doing the AI API 'right' would require a supercomputer, if you take it far enough.

A guy can dream though. Maybe doing high-frequency trading would get you most of the way there.

IMHO, go for the niche: elsewhere in this thread Directed Edge was mentioned, and they are a good example of a business tackling a well-defined problem with exciting tools and techniques.