Cyborgs and kicking are not unique, well atleast not anymore. Also both Chopper and Luffy fight with their body.
What Margret does nobody else in the crew does. Whether there are a group of ppl that do the same as her shouldn't make a difference. I mean other ppl use swords to fight does that mean Brook shouldn't have joined?
Sorry for the very late reply. The thing is they still a have a style of their own. Zoro and Brook use swords but in different manners. Also, the Franky Family are not cyborgs except for Franky of course. No one uses(none yet) rumble balls except for Chopper. I won't enumerate them all but I think the only flaw I can think is that Sanji's mentor kicks a lot too. The half-giant amazon uses her boobs to fight… should she join?
Another underlying theme/formula that I observed from the recruited StrawHats are they are all evil(arc wise) in one way or another. To name them all: Zoro - pirate hunter; Nami - thief/despised by her village; Usopp - village liar; Sanji - rude to male customers; Chopper - monster?; Robin - BW's VP/Ohara's demon; Franky - underground boss; Brook - undead; Camie - eaten a lot by monsters(very evil); Margaret - honest amazon/asks for balls(beats down Camie). If Oda will cut that formula, why now?
If we apply the evil theme to current arc, then Hancock has a higher chance than Margaret to join for being a bitch. But that would also apply to all the previous evil villains that we have before. I hope the elder joins. So far, the StrawHat family needs a grandma.