So far this is all I've thought of:
Judas Priest - seems like they're asked for every year and Bloodstock ask them every year, and fail every year. They're probably unlikely now that Nightwish have the Sunday slot and probably aren't all that cheap.
Helloween - the Pumpkins Reunited tour with Michael Kiske and Kai Hansen runs in Europe until mid-December then they have a few shows in Japan in Spring. I can see them continuing the tour into the summer with the European festivals, particularly Wacken.
So who else is a possibility for the Friday night? I guess Slayer might be available again.
