I seriously hope we CANNOT tame a dragon. That would be terrible for Dragon Age for multiple reasons:
First, dragons are a highly advanced, intelligent, and rare species. The most powerful ones, known as the "Old Gods," were even worshiped by the inhabitants of Thedas pre-Andraste. The last thing we need is the supposed "Herald of Andraste" using a dragon as a pet to play Pokemon battles.
Second, there was another, perhaps unknown, fantasy RPG that came out not too long ago that had lots of dragons and you could even call upon a few to aid you in combat. It went by the title of Skyrim I believe? Anyways, BioWare should really try to separate itself from other franchises as much as possible. "Taming" dragons in Skyrim at least made a bit of sense since you were Dragonborn. Why would the Inquisitor have the capacity to tame a dragon?
Third, I don't even see how BioWare could justify this from a gameplay standpoint. Honestly, while a dragon (as depicted in that trailer) may come to the Inquisitor's aid, it's likely fighting of its own volition. I doubt I'm the only one who'd like to keep the Skyrim out of DAI as much as possible. The game already copied enough from Skyrim as it is, for better or for worse.