I've just completed the stat blocks for all of those Elementals, so I'm now off the critical writing path. In terms of when it will be complete, John and Colin have to finish their portions and we need a full editing pass through the manuscript. As we know what all the creatures are now, it will be possible to commission the art at any point, which means we will try to organise that as soon as possible. I am not willing to make any statements about projected dates yet.
Best wishes,
Nicholas