Spoilers probably below.
I'll make a bold prediction that Oda adds more than one crew member from the Dressrosa arc. My top guesses (in order) being:
Viola
Leo and/or Mansherry
Bartolomeo
Viola is my top guess since I feel she has two really top notch skills that fit almost perfectly with the current crew composition (lookout + dancer). Practicality and entertainment potential to the max, not just for the crew, but for the show as well. An always-on-call plausibility for the anime to show breasts in movement? Not really what I need from the show but I'm sure the show producers are salivating at that. Her biggest strike against her possibility of joining is unfortunately how similar her visual design is to the other females of the crew. Originally I thought that her eye skills were not combat fit, and that that automatically wrote her off, but strangely enough they've shown how they can be used in unique ways (whale tears that smack people, maybe even haki infused?) and how much potential they can provide in the future. I just think that a proper lookout is such a glaring/stereotypical need for any pirate ship at this point, yet alone one going to sail in the New World. Also, Brook to play the music, Viola to lead the dance!
A dwarf joining the crew is my second guess, with Leo being the default top choice and my guess that Mansherry would have to at least be a semi interesting character with how much they've foreshadowed her (even though her crying baby face moment is not a good way to start). I have a feeling that Oda is still harbouring his idea of having a dwarf join (even if chopper stole most of the original guy's art design), and I don't think it's a pure coincidence that dwarves are getting hyped up for their horticulture skills (something I'm sure the crew could benefit a lot from ie. Nami's Orange Trees, Sanji's cooking, Usopp's PopGreens, Chopper's medicine). Also I figure Leo must be a pretty good tailor. Maybe Mansherry joins and Leo comes with her as her knightly protector?
Third is Bartolomeo. To me he has the best character/visual and personality design from this arc, and I get the feeling Oda has invested more time behind his design than anyone else. His ability has started to be fleshed out more and more (limited surface area) and really, he has just extremely great synergy potential with almost all the members outside the "monster trio". The story has given him an abnormal amount of screen time in battle/interacting with the crew and to me it would seem very weird thing to just snub him. His most likely "out" from Oda getting him to join is due to already having a crew but /shrug, if Franky/Usopp can leave behind a group to join, why not Bart? He most likely unfortunately just captains a supporting ship that goes out and "recruits" other ships under the Strawhat banner. I mean, his crew mate's tag was "The Missionary"… :(
But ya, who knows, maybe Oda just says screw it, and at the end of the arc Vivi flies in on a grown up Carue who breathes fire...