Not likely to impact Standard, Fortress is a very interesting card nonetheless. EDH players have weighed in online and Ghave, Guru of Spores players have already dismissed this card as something they aren't super interested in. Even within the EDH community, people have big blind spots where their own experiences don't give them much insight into what players of other decks might want.

So the price of this can never hope to exceed the price of Prophet of Kruphix unless this got played a TON in Standard. I can't say it won't without having seen the entire set, but I'm reasonably certain that this is a dud. It's a cool effect, but this will be a very affordable rare and I wouldn't get too excited about it.