People have been saying 2-3 more members.
One more permanent member for sure. And the last spot could be a guest spot that rotates people out like Vivi, Ace and Zoro's kōhai. (fuck I miss those 2 guys)
And I am saying it again last spot will be filled by a loli ninja girl that has some kind of DF power, she can be the new spy so Sanji can focus on fighting more.
There is no real positions left. The musician was the last one pretty much. Cleaner/maid, lifeguard, spy, food gatherer, tailor, artist. In one way or another the current crew does all this already.
But I can see Oda turning any one of these into an awesome role on the ship,