Ok, first off I wouldn't make this choice myself. Its one of those drastic changes that make the HTTYD world feel a little less real (maybe, I'll give my counter argument later). It has not been confirmed whether or not this new dragon is albino (no eye color has been noted as of yet). I must say a white dragon brings up a pretty distinct feeling of fan-made stories and art, and not in a good way. Does this mean Night Furies come in all sorts of different colors, or is this a very special individual who is albino? What does this major plot point say about the upcoming film?
First off, my gut response is to panic and think it is dumb, but that was my exact response to some of the dramatic changes that were announced for the second film. I worried that a dragon/human war would turn HTTYD into a shallow action series. I also was seriously concerned when it was announced that Hiccup would have a flaming sword and wear "biking-leather". However, at the end of the day, HTTYD2 was a GREAT sequel. If it had not taken the bold steps that it took (aging the characters, updating Hiccup's look, having Toothless glow, killing Stoick), the film would have certainly felt like a shallow remake of the first film. This is the trap that many animated sequels tumble into. They play it safe so as to ensure the brand continues and the money keeps rolling in. HTTYD has been taking bold steps ever since the first film, so just because something is shocking and different, doesn't mean it is bad. I also have grown massively in my respect for Dean as a director. Before the second film, we had no idea what to expect from new HTTYD material. We had seen one excellent short (GotNF) and a whole lot of mediocre tv episodes. That didn't leave me feeling confident. However, after seeing HTTYD2, after hearing what Steven Speilberg had to say about the script, and after hearing so many members of the cast praise this third film's story... it becomes easier for me not to panic.
I do hope she is an albino. I would not like it if such a drastic change in color was normal for a stealthy species. However, if you look at this new "White" or "Light" Fury with a positive lens, it might be really, really cool. Albinism is a pretty intimidating look, and with the high budget graphics of this next film, she could come out looking really, really cool. Just imagine a cross between the I-Rex of Jurassic World and a Night Fury! She could look both beautiful and intimidating! Plus, from a design perspective, it will make it very easy to visually distinguish between Toothless and this new dragon. They might look visually stunning when placed near one another, and from an artistic standpoint, I could see this working really well.
I for one would freak out, but I still have high, HIGH expectations for this film, and deep down I am confident that it will deliver. I'm excited to see what the trailer will show. On one last positive note, those who watched the clip described it as being reminiscent of the Forbidden Friendship scene from the first film. That is both gorgeous, and kind of heart-wrenching. This new dragon might be one of the main causes of Hiccup and Toothless's separation in this third film. The tension and anxiety that hearing this gives me is a GOOD thing. I want this last film to test these characters, test them even more than the first film. Now it won't be the deep, instinctive, red rage that drives Toothless against Hiccup and causes Hiccup to push Toothless away, it will be a deeply motivated conflict. They will both be struggling as both their worlds pull on them, and I think that will make for an incredible plot. Sure, it makes me worried, but that is all part of the brilliant story-telling this franchise is known for!