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.