It's the sort of thing that changes everything. All of a sudden, free will is a lie. The Master Chief's accomplishments aren't the result of his choices or his ability or the efforts of the people who trained him.

It's prophecy and predestination packed up in a pseudoscientific parcel.

: Cortana's
: plan is great because it would explain why she's always been a little bit
: special even with her unique birth.

She was created from the cloned brain of one of the greatest geniuses in humanity's history, programmed with every intrusion and cyberwarfare algorithm ONI developed in a long war, a fragment of her AI matrix perused the Forerunner equivalent of a wikipedia database prior to the fall of Reach, and then she spent hours roaming free in Installation 04's mainframe.

How much more special does she need to be? Should three Precursor wise men have shown up and paid tribute the moment her compiling was complete?

There's a point where you have to quit making your protagonists intrinsically special, and let their actions and accomplishments speak for themselves.