According to recently released research papers from Microsoft, it looks like adjustable, Xbox-style grip controllers could be used for mobile gaming in conjunction with smartphones and tablets.
As reported by Windows Central, these research documents have just been released this month, though it appears the research itself was conducted between 2012 – 2014. The key design goal for this mobile controller was “to combine a compact and convenient-to-carry form factor with the ease-of-use, control and comfort of a traditional physical controller” by using two hinged “clip-on” controller fragments that attached to the bezel of a mobile device.
Source: IGN.com Microsoft Could Be Bringing Xbox-Style Controllers to Your Phone