Personally, I imagine defeating the Gorosei will be more like exorcising a ghost than a traditional One Piece battle.
If the Gorosei are some form of immortal, then they probably have something keeping them alive. "Unfinished Business", if you want to get poetic about it. I think defeating the Gorosei will be something more like finishing their unfinished business or bringing their souls true peace in some way. The Gorosei don't seem like very happy people, despite ruling over the entire world. I think a battle between "those who smile, even while facing death" and "those who NEVER smile, even when they have everything" is going to be thematically important in some way. I think their powers are somehow tied to lingering resentment or grudges, like a ghost, in a way.
Think of it as another angle on "People only die when they're forgotten". The Gorosei will only fade away and move on once they forget about their reason to stay on this world.
