Hasn't this been asked before? Nevermind.
Whatever it is it'll have to be some kind of "concept" type. Fire, Water, Grass, Rock, Ground, Steel, Ice, Poison and Electric are all "substances" in that the Pokemon are partially or wholly made of this type. Bug, Ghost and Dragon are similar in that they are just what they are. These cover pretty much everything physical you can think of (all plants are Grass, liquids are Water, hot things are Fire, bugs are Bug, etc.) Normal is a kind of special category for anything that doesn't fit anywhere else or for Pokemon that are similar to real animals.
Flying, Fighting, Psychic, Dark are "concept" types in that they describe what a Pokemon does (Flying types fly, Dark types play dirty, etc.) IMO, this is the kind of type a new type would have to be without it becoming silly.
Maybe Captain Planet wouldn't mind there being a Heart type.