Would you want it voiced by Dee Bradley Baker? I mean, canonically, why not? But I suspect 343 wants to stamp its own mark on the voice of the Flood.
Why would a Gravemind appear in the new games? Because it's pretty damn obvious the Flood will be back in Halo 5.
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So burnt out from the Flood. Makes it feel like they aren't very interested in expanding the Halo campaign experience.