Since the mystery card isn't Heroes' Reunion according to MaRo and it does something new, would it be entirely unreasonable to hope for a Planeswalker variant of Eladamri's Call? So far, with the exception of the new Ajani, we don't have anything resembling a tutor for Planeswalkers.
If i remember correctly MaRo's answer also included that the new card had the mechanic strive on it ( we did not know what it was called at the time ) so doubt it is a planeswalker variant of the call.
Momir Vig, Simic Visionary
Melek, Izzet Paragon
Oona, Queen of the Fae
Bruna, Light of Alabaster
Gisela, Blade of Goldnight
Rhys the Redeemed
Jarad, Golgari Lich Lord
Sen Triplets
The Mimeoplasm WUBRGSliver OverlordGRBUW WUBRGSliver Hivelord(Superfriends)GRBUW
Mogis, God of Slaughter
Daxos of Meletis
so it has to have Strive or Constellation.
(my money is on Strive)
Reprint Stasis!
Control needs more love.
EDH:
Momir Vig, Simic Visionary
Melek, Izzet Paragon
Oona, Queen of the Fae
Bruna, Light of Alabaster
Gisela, Blade of Goldnight
Rhys the Redeemed
Jarad, Golgari Lich Lord
Sen Triplets
The Mimeoplasm
WUBRGSliver OverlordGRBUW
WUBRGSliver Hivelord(Superfriends)GRBUW
Ye gods I hope it has constellation.
Strive is just... no.