Yeah, there should be no problem adding the extra two sacrificial stones. There are lore stones around the map that specifically mention seven Foresaken. They could've added two blank stones as placeholders. I'm guessing they held off because they didn't know exactly what the outline of the final two bosses would be and didn't want to have to redo the stones. Interestingly enough, unless something changes in the future, this to me sounds like they plan bosses for the Mistlands and I assume the Ashlands but nothing for the Deep North? I can think of appropriate bosses for the Ashlands AND the Deep North (Surtur and Laufey) but no idea what the boss for the Mistlands could be.