I thought his voice at the end was him reflecting on the humans kicking his butt during their war 100,000 years prior. I don't recall the exact wording, but I assumed this was that or perhaps the weird "second form" thing the Forerunners take on when they die (Bornstellar talks about this in the beginning of Cryptum).
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343i said Chief will have an archnemesis, a foil character to him. I can't see that being anyone else other than the Didact.