[sayde]

About the only thing I'll remain skeptic on is what they need to open the gate/seal. While there's enough to suggest they need the power of a hollow specifically, I can't help but wonder if the only thing they really need is an extremely strong spirit. Because Shuren mentioned nothing of needing the power of hollow specifically. He only said that Szayel and Aaronierro's level of power was nowhere near enough to open it. This implies that those two could've indeed opened this gate/seal if they were strong enough. And since those two should no longer be hollow (since they were killed, sent to hell, and have now become "sinners"), it leads me to question whether Ichigo's perfect hollowfication form is truly the only viable subject they can use to achieve their goal.

...unless Szayel and Aaronierro were still hollow upon entering hell and don't completely change into "sinners" until they undergo some sort-of "sinner" transformation.