My concern is with them using an established character and turning them into an anthro, especially when the last we've heard of them in-story was Ajani claiming Jace was dead.
If this were a silver-border set or Secret Lair, sure, whatever. Same if they were making anthro alts of existing Planeswalkers as chase cards for the set, like Expeditions or Masterpieces. If they made LotR versions of Urborg, I wouldn't really sneed if they made a Mouse Guard version of Jace the Mind Sculpter.
My annoyance generally comes from the fact that, as far as we know, Jace is still a Phyrexian planeswalker. Of the 8 planeswalkers that were compleated, only 3 had the process reversed, and of those 3 only Ajani was able to keep his spark intact. If we're going to just bring back Jace, I would've liked to see an explanation of how he was uncompleated, or at least have his uncompleated status be a surprise reveal for this block.