I don’t think, there are any negotiations with SOAD for GMM, it’s just too late. Especially with Saturday headliner released and full lineup of HF on the Tuesday. As their tour announced a month ago for majority of fests and already released 2 GMM headliners, I see only possibility of HF with the whole line up, as it was last year with TOOL. GMM they definitely wait for 2 big acts tour announcements, or 1 big act + Volbeat after. Don’t believe in AC/DC due to their own stage, but looks logical for Metallica to play EU between US festival dates.
I'd rather have SOAD than Metallica to be honest. I've seen Metallica 3 times already, yet I've never seen SOAD before. Plus, I think Metallica would draw in a lot of fans that wouldn't come to Graspop if Metallica weren't playing there. Not really Graspop/festival people if you know what I'm saying.
But this is all very subjective. I'd get it if the Graspop organisation would like to have a band like Metallica on their line up, especially for the 25th anniversary of the festival.