I don't want someone too old for Jennifer Walters. Definitely not someone like Angie Harmon. If they were going to go for someone in their mid thirties, I'd prefer a young-looking one. In the first and maybe second season of Ally McBeal, Calista Flockhart (before she got too thin) did have the look of comic Jennifer and the kind of humour required. She was also fast talking too and had her share of verbal sparring with Robert Downey Jr in season 4 I think it was when he was part of the main cast.
Today though, I don't know exactly who. I'm half thinking someone like Alison Brie, but sometimes her voice is a little annoying and some of her mannerisms too. However, she is 30 and can do both serious and comedy. I think she fits into a Chris Pratt mould. Not saying it has to be her at all, because ideally I'd prefer someone else. But someone like her rather than a standard tough chick (ie definitely not someone like Gina Carano).
And here is the Movie Reshape technology they should use:
Of course the actress still needs to be fit and in shape, but Movie Reshape can make her taller, more muscular etc and enhance all of her features while still looking natural, and most importantly of all, still using the actress without the need for motion capture.