I don't think it's a fad.
Rune was Norse mythology even in it's fist installment, so naturally, it will continue the theme. Ragnarok is an important, massive event, so including it in the game would have been an obvious move.
TQ already explored Greece, Egypt and Eastern mythologies. Norse, again, is an obvious choice, with Ragnarok being the biggest catastrophe event in their myths. Since TQ is built upon disasters, which out hero is trying to prevent, naturally Ragnarok will be included.
I don't care about Thor, though. Don't watch capeshit.