When the DP1.4 officially exist on a Mac, then release a 5K Cinema display should not be a problem.
However, at this moment, I doubt will Apple release a monitor that with eGPU. Of course, that GPU only has to drive the pixels, but no need to do any 3D work (I mean on Apple's point of view), that should be very easy to achieve, and no one can blame it's performance (as long as it can drive the 5K Cinema display at 60Hz).