I'll check it out. Is it on Netflix by some chance or would I need to buy it? Muscle mass is not important. Just ask Chris Hemsworth or Chris Evans. Both of them packed on a lot more muscle than Carol will ever need. Emily Blunt, for that matter, went from a twig who was one flu away from her ideal weight in The Devil Wears Prada to the woman we saw in Edge of Tomorrow. The question is (and in my opinion always should be) "Can this person carry the film/franchise?" I don't care about whether they have martial arts background (Ronda Rousey can fight, but could in no way carry a film and neither can Gina Carano; although she's been "ok" in a couple of films I've seen her in).
She'll be getting paid to act, not fight, not have blonde hair, etc. All those parts can be handled. Can "she" (whoever "she" is) act? It's true that some actor's general on screen persona may fit better than others (certainly we can't have any twisted smiles

), but good actors can also surprise you. I always used Heath Ledger's performance as The Joker as an example. That performance for me came out of the blue. I knew he was a very talented actor, but I in no way saw him as The Joker and would never have imagined him putting on the performance he did.
I'm intrigued by some of the suggestions I read on this thread and try to keep an open mind about this (although I know Emily Blunt would do the best job

).