The first movers on the iPhone were making flatulence simulation apps and flashlights and IamRich. The bottom was empty and there was a bit of money there. But even in 2008 a developer would probably have been better off building something for the web, just as in 1995. HTTP hasn't changed and neither have the rules for publishing to 'the internet app store'. To a first approximation, all the huge profits from the iPhone have been had by Apple: it's hard to think of any other billion dollar business built on the iPhone. Even the B2B iPad loving Plangrid and DrChrono don't appear to have hit unicorn velocity.
So if I had to place an optimum bet, B2B on the internet. The 'new' platform that I think has legs is Windows 10. Between developer tooling and a roadmap that suggests a goal of running on just about anything with a microprocessor and 'integration with the Cloud' resistance may turn out to be futile.
Good luck.