Monet would have been a very nice addition to the crew, as she was not only a female but also a Logia. And her design was pretty unique. The only problem was her personality was too similar to Robin’s, I guess.
Bartolomeo, because I like him and his powerful ability would have come in handy in many situations. He would have had great chemistry with the crew, also.
Kaku would have been a good alternative, had Franky not joined the crew (but I’m fine with Franky joining it, he’s more unique and fits in better).
Perona, because she’s my girl.