I am a massive fan of Hansi, so this pleases me a lot. And even if you don't like him, he has a very distinctive voice and Arjen used his voice really well in the background of some choruses on 01011001. I actually would love Olbrich from Blind Guardian to be one of the instrumentalists as well (always very recognisable and melodic solos), but I think that chance is very slim.
And yes, some new, unknown singers would be neat as well. Maybe he will use some talent of the epic rock choir from the theater show. I know they aren't exactly "new", but some of them are definitely relatively unknown talents with barely any recorded material out there.
Furthermore, I also hope for two heavy hitters that were in the Netherlands recently, though both are very, very unlikely. One is Bruce Dickinson and the other would be Alice Cooper. I know the latter one was on his list but was way too expensive, but who knows these days.