Starting with this set, all planeswalkers past, present, and future will have the supertype legendary. They will also be subject to the "legend rule." The "planeswalker uniqueness rule" is going away. What does this mean? In short, everything that's true about legendary creatures will now be true about legendary planeswalkers.
So you could have a Jace, The Mind Sculptor, and a Jace, Telepath Unbound in play at the same time. This seems legitimately Vintage-changing to me.