Oh man... Van Tomiko would be AWESOME to do a duet with for Ayumi. I think Ayumi's pop voice and Van's rock voice would mesh very well.

BoA would be really good too. Except I'd think she'd try to out-do Ayu.

But I think definately Kaori Mochida would be the best pick. Kaori's voice just soars when she sings certain songs, like "Kiwoku"... and I think if they did a moderate song, they would sound awesome.