In the books the Spartans has unbelievably fast reaction times. They were able to occasionaly dodge bullets. What if in H2 you were able to dodge bullets when your adrenaline spiked. This would be bad ass.
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You'd mess up the game because everyone would think about Halo trying to copy the Matrix. Anyway's when would your adrenaline spike? Unless you connected the game to your nervous system (system that runs your nerves), it won't work.