I swear Arjen has already used all the good singers. Every singer I think of that I would want on this album has already been on an Ayreon album. :/
I could name a bunch I'd love to see (going off the top of my head, so I don't know if they had any bit parts on earlier albums).
Johnny Gioelli, Mark Boals, Altzi, Kelly Sundown, DC Cooper, Dec Burke, Alder, Matos, Edu, Tarja... lots of possibilities