User blog comment:Fallenbladr/The Soul King's True Purpose/@comment-7384358-20140131073048/@comment-26420452-20140131073855

It's made clear that the Special War Powers are the people who could change the course of the battle, not the ones whose souls Yhwach intends to absorb.

However, he can (and from the panels shown, he wants to) absorb Ichigo's soul once he dies. He was able to absorb Masaki's powers through Auswählen, even though she had been Hollowfied. That means that Yhwach doesn't have Hollow immunity problems.