I'm not sure it'll be a complete, or design-focused replacement.
This said, GitHub have been very good at maintaining a smart choice of features on top of git's core. That makes me believe some improved design files support could be in their plans.
There are already some simple image diff tools in github's UI, hopefully they'll expand support for more complex file formats like Photoshop and Sketch files. Or launch a designer friendly environment for this type of usage.
I'm not sure it'll be a complete, or design-focused replacement.
This said, GitHub have been very good at maintaining a smart choice of features on top of git's core. That makes me believe some improved design files support could be in their plans.