Early in my startup career an investor told me, "There's money everywhere. Where you go, how you get there, and how fast, is dependent on your skills, drive, network, and luck. But there's money everywhere. Never forget that."
For me, the biggest untapped market potential is educational video games (which is why I work on supermathworld.com). The market literally doesn't exist. There are but a handful of educational products that could rightfully be called "games".
I'm not so sure about the "literally doesn't exist" part, schools have been buying educational video games for decades.
25 years ago I played educational games that look conceptually identical to the kind on supermathworld and mathbreakers. I built space stations, launched rockets, battled monsters, and even learned geography and critical reasoning skills to catch the elusive Carmen Sandiego.
Graphics and gaming capabilities were a bit different on the Apple IIe though :)
A friend of mine organizes a conference on educational games which sounds up your alley, check out: http://intentionalplaysummit.com/
