I think the new strawhat could be someone who hasn't appeared yet, someone from the new ark who has eaten a smile and is half human-half animal or has certain mutation because of the side effects of the artificial devil fruit.... But who knows? It could be anyone.
Aokiji is very cool and has a strong power, but I have a feeling that, in his heart, he will always be loyal to the Marine, even after he had been expelled from that organization, and even if he desagrees with some marines's politics or desitions.
I think Luffy's crew will grow much larger because as all yonkous have big armies then Luffy will need a big strong army too, so I think there will be several pirates of this arc who will give their troops to him (as Chingao tried to do). Though I agree I would love to see Fujitora in Luffy's crew because he is blind, a gambler, funny and totally badass.
Somehow I'm thinking that the next strawhat would be Aokiji. Aokiji has left the marines and wandering somewhere, might as well he joined strawhats. because he's been on the 'good side' of marines and lost to kizaru already.. he should have some dark grey or white stories behind as well. he must have something he wants to set right.
If not, Looking back on the background of luffy's crew, they were ALL victims of injustice in their own circumstances (u should get what i mean by rereading from ch. 1). If someone fit that shoes, then he/she will be the next strawhat crew. Ofcourse that will not be the only factor. They must be diversed and well fit with Oda's sense of humor. Who knew fujitora might be the one who joined next lol xD with his dice rolling tells him to join strawhat lmao xD if strawhat is lacking a certain type of people in the crew then it would be an old blind gambler rofl (currently: boozer swordsman, beauty thief, trickery sniper, womanizer cook, human reindeer, elusive archeologist, sensitive cyborg, and a zombie bones)