You don't choose creatures until the ability is resolving
Of course the official rules should always be the base to start upon and they can be found here, but even those rules can be modified by playgroups according to their needs and can lead to much better gaming experience
Deathtouch Another way to discourage blocking is by giving a creature deathtouch, especially if the creature is small and will just come back from the dead anyway like Bloodsoaked Champion or Reassembling Skeleton or from Ravos, Soultenders triggered ability
A player prompted to name a card may name the combined back face, and each player has the right to know that combined back face's characteristics at all times
Price of Progress Fittingly, the more powerful and greedy the decks at the table, the higher the Price of Progress
If that spell's mana value is 10 or greater, Zaffai, Thunder Collector deals 10 damage to an opponent chosen at random