One of the limiting issues with the digital engines in the Prologue (and likely with the XD too, though we don’t know) that limits the stuff that can get ported to it is the very small amount of memory with each processor. That means that algorithms that are memory intensive, versus processor intensive, are more difficult to do – or impossible. (Like for instance sample based synth approaches.) The other issue, though less of a problem is the limitations on the user interface for the control of the digital side.
The possibility exists if the xd has a processor per voice like the Prologues, and follows their protocols, for Korg to improves things, for instance with greater memory, or a more extensive user interface. That could mean the xd would be able to have even more content created. I really don’t expect this to be the case – though hopefully the multi-engines and interface are not less (individually) than the ones in the Prologue.
The xd might be a less expensive entry for those who’d like to develop code for these engines.