I concur with you on Storm the Ivory Tower. Given that the Fortress remains 3x3 the obvious rule would be that:
orthogonal pieces can move diagonally one step to/from, or two straight through, the centre of the fortress (existing Jang Gi rule);
diagonal pieces can move orthogonally one step to/from, or two straight through, the centre of the fortress (central orthogonals of Fortress).
Compound pieces would of course be unaffected.