Definitely Yang if she switched with Blake. Yang is this bold, in - your - face fighter. But as Blake, she'd have to be more reserved, more about stealth and mystery. It would be fascinating to see how she'd handle that. She'd have to suppress her natural urge to just charge in and instead think more like a cat - burglar type of character. Plus, she'd have to deal with Blake's own personal issues in a different way.
In my opinion, Weiss would be the most interesting in a 'rwby switched roles fanfic' if she took on Ruby's role. Weiss is used to her strict upbringing and structured life. But as Ruby, she'd be in this more laid - back, go - with - the - flow kind of environment. She'd have to adapt to Ruby's more impulsive nature and it would be really cool to see how she'd balance her own refined nature with Ruby's more free - form style.
I think Ruby would be really interesting if she took on Weiss's role. Ruby is so used to being the free - spirited leader, but as Weiss, she'd have to deal with all the expectations and formalities that come with being an heiress. It would be a huge change for her character and would force her to grow in different ways.