Do you remember Klabautermann? ,,is said to be a water spirit (or fairy) that dwells on ships and is basically an incarnation of a ship that has been well cared for. It is a merry and diligent creature that is said to warn sailors when a ship's in danger and help in bigger ways sometimes.''
In other words, the embodiment of ship, in that case, Going Merry.
So, what this has any to do with Im?
I beleve, that Im is embodiment of goverment or throne or something else connected to Void Century. I think, that he will don't fight with anyone, but rather he will be defeated with some unconventional way. Do you remember Tom Riddle from second Harry Potter movie?
Maybe Im will be kill when throne will be destroyed? That would explain, why nobody can sit on it.
PS.:Sorry for bad english :/