If they are introduced they will absolutely be night elf for the alliance and blood elves for the horde. Their “sharing” comes in their story and identity.
Darkfallen are very important to the horde based on their importance to forsaken and blood elves. This importance has a lot of story hooks that most likely will not be explored for parity sake.
I don’t believe that these darkfallen developments are good for playable san’layn.