If a recent patent has any say on the future, Nintendo may be working on a playable Game Boy phone case that would work in tandem with your touchscreen smartphone.
Filed on March 16, 2018, this patent describes a phone cover that would enclose a smartphone and, via Silconera, “one of the key features of this Game Boy casing is the ability for inputs on the buttons on the casing to be sensed by the touchscreen beneath the buttons.”
The familiar set of buttons would “use a conductive sheet within the cover that enables the touchscreen to sense the input when the nub on the bottom side touches the touchscreen, in the same way human fingers are sensed.”
Source: IGN.com Nintendo May Be Working on a Playable Game Boy Phone Case