SUBSTITUTIONS FOR STARCH: You can substitute potato starch for the cornstarch, just know that potato starch thickens at colder temperatures, but cornstarch doesn't achieve full thickening power until it is boiled so you won't want to get it as hot. Arrowroot starch won't work for this recipe because arrowroot, when mixed with milk, turns slimey.
SUBSTITUTIONS FOR SUGAR: I haven't tried honey or maple syrup in mine, but I would imagine it would work well.