PKP had a lot of rock on the main stage at sunday in 2019 with Prophets of Rage, Airbourne, A Day To Remember and Pennywise. It also had TOP, Billie Eilish and Anderson Paak though.
I wouldn't rule out Slipknot. But someone said there would be an extra day at the festival site this year. Slipknot is touring with Knotfest this summer in Europe. I can see some sort of Pukkelpop x Knotfest collaboration as the extra event.