Gaga - Artpop isn't even platinum yet and probably won't be until 2016 so lets not even discuss multi-platinum
Katy - PRISM will eventually reach 2x Platinum
Rihanna - possibly
Madonna - unlikely but its possible
Beyoncé - her latest album is almost 2x Platinum or already is
Lana - simply not commercial enough
P!nk & Taylor - should be able to get 2x Platinum albums
Kelly Clarkson - I don't know anything about her sales so I can't judge
Christina & Britney - Neither of them will reach multi platinum again. Doesn't need explanation
Adele - multi platinum is on lock, however I doubt she'll be able to do Diamond again
Demi -
Lorde - Pure Heroine is still selling consistently and I could see it going 2x Platinum