I agree with those who have mentioned Annie Lennox, Nelly Furtado, Kate Bush, Elton John, U2 or Scissor Sisters.
I'd also say Madonna (how about a groovy piano version of Lucky Star???), Shirley Manson from Garbage, Geri Halliwell or Joss Stone (they have a really low pitch, so I guess it would work with Mika's falsetto).
Among the guys... mhm... some Beatles song with Paul McCartney? R.E.M.?