I think Brooke has a better chance than a gyojin. Here's why:
The longer you get into the series, the weirder the nakama get.
(I'm speaking by order of introduction, not joining)
1. Ordinary swordsman with exotic style.
2. Woman whose primary skills are manipulating the weather.
3. Cowardly engineer/sharpshooter.
4. Kick-ass chef with deranged chivalry.
5. Woman who can sprout body parts.
6. Anthro reindeer.
7. Do-it-yourself cyborg, powered by cola.
Dropping a gyojin onto the crew is pretty much a known quantity– you can tell there will be strife with Nami, you know they'll send him in when an Akuma no Mi eater drowns or for recon, you know their douriki is in the 100-1000 range, you know their move set will be "super-Hatchi", "super-Kuroobi", or "super-Chuu"...
Franky raised the bar for weirdness. Maybe we could have had a gyojin before Franky, or before Chopper (Chopper's weirdness is slightly higher).
Brooke might have the weirdness necessary to top Franky. Skeleton. With history and powers to be discovered. Who turns the regular "Luffy drags the reluctant nakama into play" on its head. At the very least, he might be a little more restrained than Franky, who IMO got old about halfway through the Fukorou fight.