To be honest. I expected them to update it with so many druid themed creatures featured in Legion. The fact they haven't updated it by now makes me think it may not be updated any time soon. Unless they throw it into one of the later patches. It's a shame really as they made a higher quality model for the nightmare corrupted treants. I still wish the cosmetic treant form had different colouration depending on the druid's race to be honest. But that's probably asking too much. :P