Post-Grand Line, Oda tends to make it pretty obvious who'll join the crew. Except for Robin, I guess?
A talking reindeer with a tragic past and an interesting fighting style? Oh, I wonder if he'll joi-oh, look at that, he joined.
A pervert cyborg with a tragic past and an interesting fighting style? Looks like he's here tp stay?
A skeleton with a tragic past and a not really interesting fighting style? Hmm. Seems like a character of the arc and won't be join-oh, he's a musician? Well that changes everything!
Kyros has tragic past down to a tee, but his fighting style isn't really all that interesting. And his fight wasn't particularly any different from the other Strawhat and Colosseum fighter battles. Chopper got his battle with Chessmarimo, Franky got to fight a CP9 member. And Nero, but who cares about Nero? Brook lost to Ryouma, but he did help fight against Oars. Not even Lola and her crew did that.
And when you think about it, he doesn't really have much to contribute to the crew. He's a gladiator, which isn't really a job. His one-legged-ness might give us something, but not much, honestly.
Bart and Cabbage have their own crews. And Bellamy's Spring Spring powers are too similar to Luffy's Gum Gum abilities to be interesting.
I'd much rather the next crew member be female, though.
The Reviere's soon. Vivi, maybe?