From this page.

"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 permanents will now be true about legendary planeswalkers. Also note that this does not mean legendary planeswalkers can be your Commander unless an ability says so."

So it seems that certain cards, like Empress Galina or Sisay can interact with Planeswalkers moving forward. Does this open up any playable design space in Legacy?