I think Robin would fly as Gloria. That would be a very, very good fit for her for the reason @Chris B says: Joan Van Ark's Gloria was way more nuanced and Robin could do that, we saw it in her last scenes as Vivian...but when it comes to Vivian, I am not at all sad to hear she won't be back because I thought she was atrocious. It seems very odd that they would pointedly tell her they won't be having her back, unless they've already got something confirmed with Louise Sorel, which would be wonderful (though I somehow doubt it).
I still can't believe AMC thought they could get away with casting Beth Ehlers as Liza Colby. I think I would've seen whether Elizabeth Dennehy or Alexandra Neil was up for the job before hiring BE. As it was, she didn't even make for a decent Taylor, and that character was created FOR her!