I've never played warcraft so I don't know what you're talking about. But what makes you assume that these dragons can speak the human language? Why couldn't it be the other way around where the dragon slayers could speak the language of the dragons? That seems a lot more likely to me, considering the fact that they were all raised up by the dragons and babies learn the languages they hear really quickly when they're young.
I admit that some dragons may have different views and there is a possibility they could change forms. But what reason would they have to turn into a human? Humans are weak. Dragons can fly and use powerful magic. It wouldn't be loneliness either because there are other dragons and also the dragon slayers they raised up. If you're going to say because Layla fell in love with Lucy's father, well her father definitely doesn't seem pleasant at all
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Ok, there is no way Lucy could be a dragon slayer, even partially. The most important thing is that dragon slayers use a
different type of magic. Yet Lucy already has a type of magic, which is stellar spirit mage. To be a dragon slayer, you have to start out with an elemental type of magic. Unless we suddenly find out that mages can use/learn more than one kind of magic, Lucy is stuck with summoning stellar spirits. What you're proposing is like saying Erza could be a dragon slayer(which is highly doubtful considering she
already has her own type of magic that she uses), since she was also an orphan.
And while we don't know for sure why the dragons wanted them, we can make a very good guess that they wanted them in order to defeat Zeref, since it was the dragons who originally defeated him.