It would be interesting to see a younger, less mature female with no emphasis on sexiness join the crew. I like Nami and Robin, both are great characters. But I think Oda has been laying on the fan service a bit too thick lately, and someone to contrast with that would be a welcome change.
It would enhance the dynamic between the girls as well. We see the guys hanging out a lot, the girls considerably less so. Someone to pull them together a bit more would be nice.
I wonder what Aokiji's final disposition will be, and wow the possibilities of him becoming a SH Nakama, I mean Akainau is mortal enemies already with Luffy, and WB Pirates, and prolly Aokiji for they're long battle.
Aokiji will never join.
He is no longer a marine but he still has his views and beliefs.
He is also far too powerful.
I had the same problem with Jimbei, but Oda seems to be waiting to let him join at the same time the SHs take down Big Mam. By then they will be a lot more powerful and with some experience in the NW. Luffy's bounty will be higher than his as well, therefore not outshining the captain.