My predictions:
1) This set of 36 planeswalkers is going to be the spiritual successor of the Masterpiece series. But one in every pack is a great upgrade.
2) They will all have their a "special" expansion symbol that is different than the symbol for War of the Spark.
3) Cards appearing in this special planeswalker printing will not impact their format legality.
4) All will be reprints. Except that there will be a couple characters that get new planeswalker cards for War of the Spark (like Bolas). It will only be characters who are involved in the War of the Spark storyline who have not already recieved a new card in the past two expansions (Guilds of Ravnica and Ravnica Allegiance). Any planeswalker with a new card in Guilds / Allegiance / War will have that version printed in this "special set".
Happy Prognosticating!