Charles Martinet, I'd have him probably say some Paper Luigi/Mr. L lines from the Paper Mario series. Or even Doopliss lines from when he was pretending to be Mario. I imagine him having a rude or snarky version of Mario's voice when using his body. Same obviously for Mr. L/Luigi.
Kenny James, yeah same for Bowser RPG lines, and Dark Bowser. Maybe see if he could do a variation of it for Bowletta lines (I don't like how they slapped regular Bowser clips on Bowletta though)
I'd be interested in hearing Kenny James say full lines in general using Bowser's voice. I don't recall much, if any of that in the games.
I'd also probably get Charles Martinet to say some of Mario's lines from Mario vs. Donkey Kong on GBA, even if he did the originals, I'd like to hear them without GBA sound compression. And I might ask him why we haven't heard that kind of thing as commonly in recent years, like if Nintendo's been asking him to limit his voice work.