The two biggest for me will be an App Store for the AppleTV. This could put the end to Nintendo's console hopes forever and open up a lot of new revenue streams. And the biggest by far is payments. iBeacons is starting to take off and with AppleID/TouchID they have the frictionless security mechanism.
Surely there is untapped potential in giving people greater day to day insights into their health, and obviously a lot of people care about their health.
Until it can analyze sweat for illnesses and nutrition info that won't get Apple in trouble legally (FDA...) and will spare you from going in for a check-up, then I don't think it's feasible at all for them to venture in that area.
MP3 players, smartphones, and tablets were all small "niche" markets before Apple entered them.
The only viable one I can think of is wearable tech, and definitely not a damn watch. I think Apple won't cater to as niche a market as wearable medical, and let 3rd party use their hardware to attack that market space.