Having worlds be in "inventory groups" would be extremely powerful. All worlds in the group would share inventories with each other (like the Overworld, Nether, and End do in vanilla), but not with worlds in different groups.

Say the overworld is in group Alpha, but you also want to have a minigame world in group Gamma. If the mod could control which inventory is loaded in which world, it would allow for smooth transitions and no need for a manual inventory transfer in a "waiting room".