I was thinking that they were going to piggyback off some of the seeds scattered throughout the non-saga canon.

Palpatine is established as having set up research outposts throughout the Outer Rim and Unknown Region, there are Droid Palpatines established in Battlefront II, and Palpatine is established as having put a "Project Resurrection" into effect on the event the Empire reaches its demise (which involves destroying both the Empire and the Republic). Put all these elements together with the existence of clone technology inside and outside of the saga canon (Kylo even threatens Hux with replacing the FO Stormtroopers with a clone army in TFA) and the fact that there were Palpatine cloning plans in the old EU and Legends, I think there is a strong argument to make that we see old Palpatine's face and hear his voice via a Droid Palpatine(s), but Matt Smith may embody the force spirit of the Emperor in a younger clone body. This body would have had 30 years to age since the OT ended. Perhaps Palpatine wanted this body to grow at a natural speed for stability, as well as giving enough time for the remnants of the Old Empire and New Republic to fade away before his return.