The iPhone 6, which is presumably going to be announced next month, is also presumably going to have a larger screen — 4.7- and/or 5.5-inches larger according to rumors. When you get towards 5.5, you become as much mini tablet as you do big phone. Apple already makes a mini tablet, of course — 7.9-inches of iPad mini to be precise. It's widely believed to be the more popular of the two iPad models, outselling the 9.7-inch iPad Air. So, could a bigger iPhone cannibalize the smaller iPad? Or could it lead to an entirely new market for the Air, and the rumored iPad Pro?
Apple has repeatedly said they'd rather cannibalize their own products than have someone else do it. However, there's likely more to it than that. Currently the biggest iPhone you can buy is 4-inches. If you want a larger screened iOS device to travel with, iPad mini fits the bill perfectly. It's identical to the iPad in all almost all ways but size and price, and size is the major differentiator. In a world with a 5.5-inch iPhone, however, people can have both an iPhone and a larger screened iOS device all in one. That will eliminate the need for an iPad mini for some portion of the customer base.
Yet it might also open up the desire for an iPad Air or even a — currently speculative — 12.9-inch iPad Pro.
If having a small iPhone leads to a preference for having a small iPad, perhaps having a large iPhone will lead to an equal and opposite preference for having a large iPad. Absent the need for high portability, size and power could become more important for iPad customers. In other words, when you don't travel, when you leave it at home or at work, a larger screen can be more valuable.
Apple's done a great job mainstreaming productivity with their Your Verse campaign for the iPad Air. As larger phones prove valuable to a segment of the market, it's quite possible larger tablets will as well, and for the same reasons.
I'm currently traveling with an iPhone 5s and an iPad mini with Retina display. Come November, I can easily see myself traveling with a iPhone 6 and an iPad Air 2. Come 2015, I can just as easily see myself with a iPad Pro instead.