I'd say that Savatage are a pretty big sub that could sell some tickets due to not performing in Belgium in over 20 years
With Kind Diamond as marquee headliner and In Flames & Heaven Shall Burn as mainstage logo bands for the fans of different genres, I think the day would sell decently with Within Temptation as south head. A band like black country communion could work as well as another strong sub, but they seem to be quite expensive.
Graspop often has some stronger and some weaker days, and though this would be one of the weaker days I wouldn't worry to much about it. The sunday didnt't sell out the past few years either.
Myself I'm more worried about the Friday, they really need a strong sub there (or north head if FiR is the strong sub)
If they would add The Prodigy, Muse and Within Temptation I think it would be a really strong set of headliners, but I think it might be a bit to optimistic.