Ok ppl what part of BLIZZCON EXCLUSIVE do you not understand. Even IF it is a gold card (which I highly doubt it will be) unlike Golden Geblin you won't be able to obtain the "mystery minion" card any other way than buy either physically going to BLizzcon or buy getting a virtual ticket. I do believe that it will be a legendary only because a player will only be able to have 1 of them in any deck at any time. If at any point in the future that the "mystery minion" become craftable in any way shape or form with the exception of hacking, I might shelve my HS decks and look else where because when you take away exclusivity of an exclusive item Blizz might as well give every WoW player the Blizzcon pets from the past; it would only be fair.
Unless they make it a boring uninteresting card then there's no reason as to why they can't just make a brand new legendary card, give buyers the golden version, and let other craft it, just like they're doing with the previous special card.
It would upset the balance of the game to make a powerful and useful card buyer only and they've talked constantly about straying away from that. So again, no, they will not be making it so you can only ever get this card through Blizcon unless it is a carbon copy of another card with new art (Or just a gold version of something that exist).