When Promising Future is played and the opponent plays Blitzcrank (moving a unit to a battlefield), does the showdown start immediately or do the Promising Future cards resolve first? Also, can a unit from Promising Future be played to that battlefield?

Ruling: Promising Future must fully resolve before any showdown starts. You cannot play units to battlefields you don't control (and you don't control a battlefield you're attacking).

Sequence:

• Opponent's Blitzcrank resolves
• Your Promising Future unit resolves
• Blitzcrank's triggered ability goes from pending to confirmed on the chain and its target is selected
• Blitzcrank's ability resolves
• Combat/showdown starts

Nuances:

• Showdowns only start when there's no chain - while Promising Future is resolving and cards are being played, the showdown doesn't start
• You can react after pending abilities become confirmed, but not while they're still pending
• Nothing on the chain will ever be pending while a player has priority to react