Kyle Baxter wrote a good piece speculating on where Apple is headed with the iPad line of devices:
The Mini narrowed the performance and capability gap as well. The Mini now runs the ridiculously fast A7 processor, same as the iPad Air and iPhone 5S. For many people, the Mini is big enough to write on as well, or make presentations in Keynote. The full-sized iPad is still superior for those tasks, and is especially superior for tasks like sketching which benefit from a larger screen, but the difference really isn’t that large. They are both quite capable devices for whatever task people want to use them for. The comparison is much more akin to a 13-inch Macbook versus a 15-inch than it is to an iPhone versus an iPad.
I agree that the iPad Air and the Mini are much more similar than the previous full-sized iPads. The gap that separates the devices is closing, and that leaves room for another device to fill the ‘Pro’ position.
What is really amazing, though, is the fact that the iPhone 5s, iPad Mini, and iPad Air are all powered by the same processor. If you own any of those devices, you pack some serious computing power in a very small amount of space.