User blog comment:WeissSchnee/Yellow's Trailer Music/@comment-92.59.193.239-20130603005306/@comment-61.173.88.230-20130603031342

Now that I've zoomed in the picture, the pose looks really similar--but I still doubt it. For one, if you're right about her being from another series, RWBY is going to need a really good team of lawyers to avoid being sued the ass off. Second, the theory makes sense: why else would Yellow's trailer play a remix of all the other character's theme song? It would be weird if the person turned out to be some yet unintroduced character.

Still, you do have a point and at this time, this is all rampant speculation. You could be right!