The devs have said multiple times that all dragon encounters are dynamic. They occur just like a fight against any other enemy.
Also, it could have been a coincidence that he killed that dragon in the trailer with a shout.
the devs said multiple times the same thing about npc's in OB, it was called radiant AI. the problem with this was that it was a trailer and not actual game play (at least not 100%)
the encounters might be dynamic but killing them might take a certain process; dragon shouts for different dragon types. either way, would they really make dragons just bigger tougher enemies with more health. bethesda isn't like that, they'll want dragons to be different from all other enemies and that will be the nail in the coffin imo if it really is really like some boss fight where the boss can be weakended with normal weapons but only killed with a speacial weapon.