Huh... makes me wonder if Sif was meant to be the female lead in this movie, and when the actress wasn't available, Valkyrie was created to replace her.
https://www.sideshowtoy.com/blog/thorsday-valentines-edition
Thor has had a number of love interests in the comics, including Sif, Jane Foster (repeatedly - he keeps circling back around to her in the comics), a valkyrie named Brunnhilde, and a few others.
So the valkyrie love angle is comic cannon, and a sif love angle is comic canon, and honestly I think they could have gone either way with this movie.
So yes, they could have gone either way. I only bring up the love angle, despite it not appearing strongly in this movie, because they very clearly broke up Thor and Jane, and perhaps that's incidental (didn't want to try to involve her in the story), but it's likely because they wanted to move forward with another love interest.
With this, though, they get to "test" another character/actress and find out if one or the other got more of a reaction from the audience.
But who knows, it's entirely possible they would have introduced her anyway, even if Sif was available, because they might be planning a love triangle in the future - always an audience favorite.