A touchscreen interface isn't ideal if you want to use your Android phone as a portable retro game arcade. If you have an old Game Boy and a Wiimote lying around, you can combine them to make your own hardware controller for your phone.
Instructables user alpinedelta offers this interesting solution. The Wii remote is used for the Bluetooth interface in the Game Boy shell:
With the use of the "Wii Controller IME" App on the Play App store, this retro controller will really control anything which lets you use the android touch keyboard as input. Although i've only tried with a SNES emulator, in theory you should be able to play and do all kinds of things with the Gameboy's buttons. The project is essentially a gutted Game Boy with a Wii remote inside. ... The limitation is really the support of your device with the Wiimote App. Technically you could control your wii with the Gameboy minus some functionallity.
Head over to Instructables for detailed photos if you would like to get that old-school gaming experience on your Android phone.