Okay, so we have Parkway Drive, Machine Head and Pantera as potential candidates for the Sunday headline slot (+ Scorpions or something).
Here are my thoughts on each of them and their likelihood:
Parkway Drive: Their show was one of the most celebrated in this year's edition, so a return to the festival might be clever, they also draw a huge crowd with their show.
However, their set is likely to be similar to the 2023 one and I believe the band might not see a reason to return to the festival that soon.
Machine Head: Same as with Parkway Drive but wasn't there a rumour that their 2023 set was originally planned for 2024? Might be that the original plan of a big 2024 headline show still stands.
Pantera: From the band's point of view a return to the festival makes a lot of sense - Phil seems to really like Graspop and their 2023 show was not quite as a headliner and they might be interested in being billed as one. More importantly, they played almost every big June metal festival this year, have limited options in terms of non-metal-festivals (because of Phil's reputation), yet they are back in Europe with no indication that their Download Festival appearance is a festival exclusive. I also think the reunion is still hugely interesting to a lot of people and Graspop can easily make a statement similar to the 5FDP one ("They're finally ready for the big job: headlining the North Stage" or something like that) Personally, I think their return makes the most sense.