For the thing that does matter, which is ACID within the same partition/entity group, I would argue that RethinkDB already does have that.
Mike Stonebraker says that there are no fast distributed transactions [1]. I'm inclined to believe him, and that's not just an appeal to authority.
[1] https://www.youtube.com/watch?v=KRcecxdGxvQ&t=54m22s