At the end of season 3, we all know hopper went missing. Dead? We dont know. Is he "the american" trapped in the russian prison? Seemed too obvious to be true. Now, watching the black widow trailer and finding out that the main villian is hopper? 

So, blowing up the "key" transported him to russia where he was imprisoned, tortured and became whoever this villain is. Absolutely insane and can't wait to see how it all plays out.