I'm not being entirely serious, mate. Just found it strange that this match-up would occur to anyone. For what it's worth, I'd pick Brook.
This is an incredibly hard fight to predict. Note that none of the experts have attempted going near it. I predict Kell, his power being the deciding factor with two great skilled slicksters
Someone who comes in bigger than Gennady whilst being ripped, can take GGGs blows so well, and land monster punches himself... yeah i'd say he was a full blown middleweight