I believe Sif was unavailable due to scheduling. I think Jaimie Alexander getting hurt during shooting for Dark World and taking out any chance of her character being a bigger part of that story pretty much killed that entire group for future films.
Don’t think that I’d controversial at all, I’d venture to say a better/cooler character than Captain Marvel.
They wanted him in Thor but he was unavailable so they cast someone else. The other guy wasn't available for Dark World so they got Levi
They recast Fandral twice before getting the guy from Thor 1 then had to recast him after that Hot Fandraltato