Can you still earn the diablo 3 kerrigan wings if you buy Starcraft 2 now?
Solution 1:
The wings are not a pre-order bonus, but a Deluxe/Collectors Edition bonus.
So if you get the deluxe digital or Collectors Edition you will get the wings.
Solution 2:
The Diablo 3 Kerrigan wings are a bonus part of the Collectors Edition or the Digital Deluxe version. You can purchase either of these now and once you bind the Starcraft II: Heart of the Swarm key to your account you will be able to get the Kerrigan wings on Diablo III.