First I think there will be a plotline involving Big Mom where she will train and use lots of Life span to become younger in a final attempt against the straw hats, and to take Elbaf by force. There her whole character will wrap up and I believe both Shirahoshi, and the Germa will be involved too. King will also make an appearance but not as an enemy.
Also before the straw hats reach Elbaf they'll have a little detour where they will meet a stranded person and he will do things that will make Luffy accept him in the crew; but he's a secret member of sword and will feed the marines intel about the straw hats but as he adventures with them throughout Elbaf he will betray the marines in the end and becomes a permanent straw hat
Also before the straw hats reach Elbaf they'll have a little detour where they will meet a stranded person and he will do things that will make Luffy accept him in the crew; but he's a secret member of sword and will feed the marines intel about the straw hats but as he adventures with them throughout Elbaf he will betray the marines in the end and becomes a permanent straw hat