Slider smartphones were pretty useful, but manufactures stopped making them because they were expensive to make and prone to breaking, but this idea is about bringing them back as add-ons, the sliders could feature a keyboard or a gamepad.
They would be made to be able to attach to an smartphone, and the smartphone would be designed to be attachable to the slider, and would be plugged directly into the USB port (so no need to use Bluetooth), these sliders could be sold bundled with the smartphone, or could be sold separately, and that would be convenient in case the slider breaks.
The slider could also feature an additional backup battery, which could be very useful if you like to play games on your smartphone.-- appdirect, Sep 18 2014random, halfbakery