Sure you could size the ductwork to accomodate a package unit later, basically the duct size will be the same if it is sized right for the load/cfm requirements, but remember that If you are using a closet as a central return, and you dont have return ducts and grilles this will have to be added when installing the package unit. But what I have done is simply pull the furnace out and cap of the floor, then You can tie into the trunkline with your package unit. There is always a way to get it done if you get a good installer out there.