I'm partially convinced Golbez is Xande or something.

The thing I am far more convinced of is that Golbez is baiting us into basically our only option to enter the Void being the Crystal Tower, which will be the stage of the climax of this story arc.
There is no more fitting tool to attempt a Void incursion than the same Crystal Tower Xande used to attempt his incurson, but failed. That failure I suspect was not a flaw in the tower itself, but Ascian meddling to cause a rejoining and remove the Allagan Empire in the same package.

Whether this whole Void arc truly wont be connected to the future expansion? I have my doubts. But either way, the Crystal Tower seems like an obvious inclusion.