since G4, especially snakeman, can stretch far more than base Luffy, I figured that a G4 version of red hawk would be really OP. Imagine
>Luffy goes into snakeman
> he activates G2
>instead of compressing his arm, he stretches it back as far as he physically can.
> he activates barrier ACoA on his fist
>he snaps it back, creating a massive explosion and generating more power than ace's hiken
Since red hawk seems to be based off of Luffy's haki, now that he has ACoA, I'd imagine Luffy would be even more powerful.
I always thought that Coby would get Monet's DF. If he got it I imagine he would use it similar to how Luffy uses the gomu gomu no mi with him creating a giant fist made out of snow, or possibly using it to make his body bigger and stronger as a parallel to G4. I also think he'd probably awaken it, allowing him to freeze people on touch, similar to how crocodile can dry people out by touching them.