Missed opportunity
In the book "Art of How to Train Your Dragon" they discuss their original plans for the Green Death dragon's design, how they were going to give it a sea theme, with jellyfish-like bioluminescence. It was cut and the final design used instead because the sea design was considered too "elegant"; it was hard to want a creature that looked so beautiful to die.
I was thinking about that and I realized: They missed out on a great opportunity to reuse the design for this movie, as the design for the Bewilderbeast!
The Bewilderbeasts are normally seen partially if not completely submerged in the water, so the sea inspiration would make sense, and since Valka's is meant to be a 100% good dragon and Drago's is meant to be forcibly beaten into its bad deeds, if the design helps the audience feel sad for their deaths or injuries then that's a good thing. And, of course, since it was the planned Green Death, presumably it was meant to be huge, like the Bewilderbeast.
I guess you could say that a sea inspired design wouldn't tie in with his ice abilities well, but frankly I don't think the final one ties into ice particularly well either. It's just white, that's it. If that's all it takes put some white streaks on the cut Green Death design.
It just makes me kind of sad that such an awesome sounding design was cut and then they missed their chance to bring it back.