Follow the crowd
I reckon there are plenty of opportunities out there for app developers. The problem seems to be that most developers are more inclined to copy something that already exists, rather than spend the time and effort required to fill obvious gaps in the market. I'll give just two examples from my own unsuccessful app hunting:
1: Mobile version of Open Office.
There are eleventy billion apps in the App Store which claim to be mobile office suites of one description or another. Nearly all can read/write Microsoft Word format, one or two can 'read-only' Open Office formats. But there are none which can read/write Open Office. Neo Office have an iOS App, but rather than provide anything useful like the ability to work with Neo Office documents on the go, it's a front end for their pointless "look we've got a cloud too!" online storage offering.
Similarly, Libre Office have adopted the Duke Nuke'm Forever approach of telling world and dog for months now that they're planning Android and iOS versions of Libre Office, while delivering absolutely nothing.
So, there's one obvious gap in the market for an app suite that would be guaranteed to sell and at a profitable price, but don't hold your breath. It's much easier for wannabe developers to leverage iOS's in-built support for .doc format and slap together another "me too" Microsoft Office compatible suite this month, than to spend maybe half a year building something innovative like a mobile Open Office suite.
Similar story here. Search the App Store for something that will allow you to print from your iPad and you'll find an equally huge number of printing apps that will supposedly let you "print to any printer" over wifi. Read a bit deeper tho' and you'll find that most of these apps either; A –actually print via a helper app on your desktop computer, or B –somehow operate on top of the free iOS picture printing plugins most manufacturers have released and channel print jobs through those, as images.
[There is a third category; printing apps by a company called EuroSmartz, which seem to have as many different printing apps as the rest of the market put together, with a confusingly mobile phone tariff like matrix of 'features', but that's another story!]
Again, an obvious gap in the market there. Who's going to be the first to bring something like a mobile wrapper around CUPS to market, which genuinely allows the user to add printers to their iOS device as easily as they can to their desktop? Ironically, there's an app called TruPrint for jailbroken iOS which does exactly this. But no-one seems to have managed to pull it off through 'official channels'.