With males, I think he should duet with Josh Groban. Josh's voice is lower and has a different feel but I think they would blend perfectly. Lots of songs work well for male duets. Josh recently sang with Sting in The Shape of My Heart and with Paul Simon in Bridge Over Troubled Water (also with Brian McKnight). Those songs would work and would lots of others, really lots of songs make good Male/male duets with the right voices.
With females, I would like to hear him with Hayley Westerna. I have the West Side Story album that Will sings the Jet song on and I couldn't help but think of how much better it would have been if Will had been the lead male instead of Vittorio Griggolo (just my taste). I would love to hear them sing Tonight together. Will would also do a lovely job singing The Prayer with someone like Emma Williams.
I also think Will's voice would also blend well with Celtic sounding singers like the members of Celtic Thunder or other singers with the Celtic sound. He has that type of pure quality to his sound. There are a lot of songs in that genre that would work for Will.
Yours, Linda Colleen