Rebecca has obligation to her people…whether they hate her or not. Besides, I'm sure that will be resolved when the truth of what happened 10 years ago reveals that Doflamingo is the real bad guy. Her dream as of now is to defeat Doflamingo...which will probably happen at the end of this arc or next arc if something unexpected happens. Plus she looks like Nami so...no.
Violet would be interesting power wise being a scout but her devil fruit power makes it too broad for their crew. Some of the mystery would be lost to us the audience. Robin fills that role enough with putting random eyes and ears on different places. Neither of those two designs or personalities really stick out.
Luffy: Strawhat, rubberman, always wants meat
Zoro: Green hair, 3 sword style, scar over left eye, loves his booze, sleeps all the time
Nami: Orange hair, money hungry, weather powers, fellow booze lover
Usopp: Long nose, very cowardly, huge liar
Sanji: Curly asymmetric eyebrows but only see one at a time, huge lover of beautiful women
Chopper: Talking reindeer...nuff said
Robin: Cowgirl look, make body parts with devil fruit almost anywhere, different face than Nami
Franky: Cyborg who wears speedos, light blue hair
Brook: Undead skeleton who wants to see panties
Bartolomeo...I don't know what to say. He is very unique, strong, and has a useful devil fruit. He has his own crew...though I am sure he'd drop them in a heartbeat to be with the Strawhat crew. How long would his fanboyism last once he was with the crew all day, everyday? I'm thinking more ally, then weird turn to villan transformation before nakama.