Exactly! I actually have a theory, albeit not a very liked one by Law fans....
It involves the whole plotline about his devil fruit. See, I always thought it was weird that Oda even came up with the immortal life surgery that never paid off in the end of Dressrosa. Obviously no one wants to see Law die, but I got to thinking:
Law's arc has been technically complete since Dressrosa. What is really his true motivation? He seems to be into taking down the Yonko, not to be pirate king. He wants to "break the gear" holding the world together, but what does that truly mean. He was showing his support for Luffy when Doflamingo cut his arm off, and hes also the only Supernova who tried saved him at Marineford.
Anyway, I think Onigashima's raid is going to go horribly. Neko and Jinbe may or may not come in this time, perhaps the big fight happens there or is saved for later, but I think Luffy and Kaido will fight, and again, and Luffy will lose badly, to the point of possibly dying. Now, Law being Law and what he thinks of Luffy may try and save his life. The Immortality Surgery may have another purpose: To ressurect someone. If said surgery can make someone immortal at the cost of the users life, is it that outlandish to think it could revive someone or save someone from an ultimately deadly injury?
Middle Acts in Kabuki's generally end with a tragedy, and said tragedy hasnt happened yet. I think Law will be this tragedy. The entire idea of taking down Kaido started with him, and it may end with him. He loses his life saving Luffy's, believing he is the person who will change the world.
Time passes, lets say another week or two, they fight Kaido at the captial, and that is when Sword and everyone comes in to turn the tide.
I think it really ties up Law's arc beautifully, and shows why Oda spent so much time on his character. But we'll see lol. I do think Onigashima's raid will be spectacular, but they wont win.