Continuing my series on mobile platforms, I now think killer mobile apps need mobile devices designed like game consoles, meaning that it has one or two slots for application cartridges. Each cartridge contains one or more applications. Cellphones would come with a built-in cartridge containing the phone app as well as others.
Adding a cartridge adds new apps. Applications have two modes: running and stopped. The Next-App button (aka Appy button) activates (brings to front) each running apps in turn. Some apps can activate itself when an event fires (phone call received). To see the list of available apps, keep the Next-App button pressed for a second.
Eventually, app cartridges will be advance to become platform cartridges, taking over the full functionality of the mobile device to offer better application platform services than the one that came with the device (i.e. easy to use app download service that auto-configures apps for your device). If you don't like the built-in calender, just stop it and run a third-party calendar instead.