Well, new pokemon are introduced in new generations, so even if the game is currently coded limiting redundant-pure pokemon types, there's no reason why this still couldn't make for a new pokemon.As interesting a double-typed pokémon might sound, and how would STAB be counted in such a case... The way the game works makes it that monotyped pokémon are those with type 1 and 2 the same, as far as I know.
All it would offer though would be entirely possible with an item (therefore giving this to all monotyped pokémon) or ability that increases type effects (including STAB).
Although, your idea for making this concept an item for every type is a great one--Eelektross would be a great holder of the Electric item under most conditions (not under Gravity, against a pokemon with Smack Down, or Moldreaker, etc.).
There's Gallade and the Musketeer Quartet. I'd like more martial arts-based Fighting type pokemon (as I'll only credit Makuhita and Hariyama, Meditite/cham, Throh, Sawk, and Mienfoo/shao as being martial arts-based, with some of the Fire/Fighting starters being questionable, given their inspiration) or, instead, maybe something rather like Sawk and Throh that is based around wrestling. (I already came up for a unique move for such a pokemon: "Grapple," a 60 BP power that is like Pursuit, except it won't double in power when the opponent leaves battle.)I also forgot to mention before, I want a Fighting type that isn't hand-to-hand martial arts based. Like something with a sword. Maybe a fighting/steel type with a sword for an arm etc. or just like a samurai/knight Pokemon. Or even a fighting/dark that was a ninja with shuriken, something like that.