I can see Issho turning away from the World Government, he might choose to follow the flow and let himself get pulled into the gravity which is the "D", Luffy and Blackbeard. He might aid Luffy in another way, fight of another chaotic neutral character and maybe, just maybe, make space for a new admiral.
He could also fake his own death and join Cross Guild. That's a fanboi theory tho lmao.