I'd love Katy & Jessie J to duet. They have similar tones to their voices but different qualities so it wouldn't be like hearing the same voice twice but they'd sound gorgeous together. Also would love to hear Katy cover Domino.
A Katy/P!nk collaboration would be great too since P!nk's gritty voice would contrast Katy's sweeter tone beautifully.
I don't know how I would feel about a Rihanna duet, but that's probably just because I'm not really a fan of hers.