Furthermore I also doubt that they are working on all new character models parallel at the same time.
Unlike as with the HD Remixes or titles like DDD which used the "old" art style, for the new KH III engine all characters need completely new models.
What we have seen so far in terms of KH III footage looks like as if they're focusing on Sora's model right now alongside the Heartless (old and new, as these also need new models due to an entirely new engine being used) and possibly Young Xehanort and Young Eraqus as all these models have been shown the most in trailers so far.
The reason we haven't yet seen any characters like Rapunzel, Hercules etc. is probably also because their models, if they even exist already for Unreal 4, are not in a status viable to be shown off to the public.
For KH 2.8 it is obvious that other characters will be in the focus and thus they have started on models for Aqua, Mickey, Yen Sid, Riku & Kairi, but they're obviously beta-models that still need some refining.
Donald and Goofy also seem to have new models already, but just like those for 2.8 they're also not really in an advanced stage.
As all parts concerning x[chi] Back Cover only show background scenery with voice overs I'm assuming that actual 3D Models for the Foretellers and Ephemera in KH III-engine do not even exist yet or they're only wireframe objects without textures so far, same most likely goes for all the other major original characters like Ansem the Wise, Terra, Ventus, Naminé, Roxas, Xion, Xemnas etc.
