As each wing is released, the cards available in each wing will also show up in your Arena picks, even if you have not earned them for your personal collection yet.
I really do not like how you have to 'unlock' these cards in arena by beating each wing. The entire point of Arena is that ALL cards can show up.
Arena players who ignore these new cards effectivly can game the Arena system, especially if these new cards are bad/situational in general [Or there is something like 4 and 5 contain situational cards which would be bad Arena picks], and have a higher chance of getting better cards that already exist.
I mean, with all the deathrattle stuff on the revealed cards that is clearly pretty situational, you can wager that at least some of these cards are going to be too situational to be good arena picks. And having 6 less cards in the pool per wing you ignore is still better odds...
And if the cards at the later wings ARE good? Then that's buying power in Arena and a disadvantage to those who want to spend on packs and such.