Looking back over the singers a moment:
Edwin Balogh Roman
Sharon den Adel Indian
Jay van Feggelen Barbarian
Fish Highlander
Anneke van Giersbergen Egyptian
Arjen Anthony Lucassen Hippie
Edward Reekers Futureman
Damian Wilson Knight
Robert Westerholt and George Oosthoek Death
Peter Daltrey the Voice
I think given their previous appearance that Anneke, Damian, Edward, and Jay can be easily signed on. Fish is a major wild card, I find his role on that album to be so iconic, but it's a damned if you do, damned if you don't situation. You get him and you're not going to get the same voice you heard on that record, you don't get him and you're still going to struggle to find someone properly stepping into that role. Sharon is still active, and dutch, so I think she's probably in as well. The rest, no clue, just don't know enough about them.