I bet R. Lee Ermey would do it, heck, Riot could probably just pay a local Drill Sgt and have the voice files within one afternoon of recording
.
Just a few things to add to your suggestions:
Taunt: Mess with the best, die like the rest.
Raise Morale:
if only GP is affected: "Here we go!"
if others are affected: "LETS GO LADIES!", "Hooah! Doubletime!"
Cannon Barrage: "Bring the rain."

Bump for new voice
I bought the skin but it needs a new voice

bumb for great justice

I will bump this to the day i die.

Bump

https://www.youtube.com/watch?v=PTJxzqcaJBs

Make him sound like that.

We petitioned for a new voice in PBE for a while before the skin came out

They didnt listen to us

I will not buy a skin that has a special forces guy talking like a pirate, and i really hope people dont buy it thinking its gonna be a cool skin because the voice destroys it

I actually think a new voice actor is in order, I barely even play GP but this would definitely make me excited to buy a skin for him.

Something like Graves but tougher and less country.

The thing is, this is a 975Rp skin, not a legendary, if they gave gp a new voice they'd have to put new voices on most upcoming 975 skins.