So the answer at present is that we simply dont know but we wouldnt be surprised if the popular Trainer Gallery cards do make another appearance at some stage.
Mewtwo awakens for the first time and makes friends with the little girl and the other Pokmon clones in the lab
Nissa, Worldwaker was an anti-control threat that seemed great in theory against U/B decks with Perilous Vaults and Ugins
Even though this is a version of Pokemon TCG, there is no trading of cards at this time
Given how much card draw there is, its not impossible that you get that ultimate on the turn after you play Teferi
If a token has to move to the graveyard, hand or any other zone, even temporarily, it vanishes forever and unless it was on its way to the graveyard, it wont even count as having died