Here's an idea I have about creating apps for iPad that use iPhone as a controller.
If you look at the popular drawing apps for the iPad, you quickly notice that accessing the controls is complicated. By far, the best iPad drawing app out there is Paper by FiftyThree. They have a lot of cool ideas for gestures (especially the two finger rewind, and the dragging-images-to-new-journals system). However, there's still a big problem quickly accessing the controls to change colors, brushes, etc, while still leaving the canvas free for drawing.
It would be nice if we moved the controls to a second screen. That would keep the iPad's canvas free of clutter while also allowing the controls to always be visible.
I created an app (just in the demo stages right now) that puts the color control on the iPhone, which can be manipulated with the user's non-dominant hand while they draw with the other. As you can see by the demo below, it's a much faster way to do it. This is a single, Universal app running on the iPhone and the iPad at the same time. It knows to connect automatically to the other device, no set up required.
The app uses Apple's iBeacon technology and Core Bluetooth to drive the communication between devices. It uses very basic Core Graphics to display the images drawn.
Expanding on the idea
There are many ways for this idea to be expanded. In a drawing app, there could be controls for brush size as well as other tools (making shapes, etc). In other categories, similar ideas could be applied to text documents (simply a keyboard on the iPad, with font and style controls on the second screen), photo editing (with photo on iPad, and filters, sliders, and controls on the phone), and more.