Hm, maybe Ditto; Ditto's only purpose is to transform into the opposing pokemon to use it's own moves against them, but on it's own right, it can be quite the counter to pokemon who'd love to use moves like Dragon Dance or Swords Dace, etc. Then again, a player could easily counter Ditto using the move substitute, since Ditto can't transform into a 'substitute' pokemon. Hm, actually, that makes me wonder if Ditto could transform into Mega pokemon - that'll be cool!
Another pokemon I was thinking of is Keckleon. Now don't get me wrong, I think it's a cool pokemon, buuuut, Color Change is a weird ability no? The ability to transform into the type that you've been hit with? Players all the time make that ability work to their advantage, for example, let's say they have a Rotom-W, and they used Hydro Pump,changing it to a water type. They can easily follow it up with a thunder type move to finish it off, and then switch to a pokemon that has a ground type move, and so on until it faints. It's not to say it's a bad gimmick, it's just, you have to be careful on how to use one!
Next I'm thinking Castform, but again, I don't think it's a bad gimmick. It has a good movepool on which it can abuse it's ability to transform to an preferred type. On it's own right, it can serve as an interesting check to opposing weather teams, like, let's say, you're facing a Sunny Day team that has a lot of Grass types with Chlorophyll - Castform becomes a Fire Type and you could use Weather Ball or another Fire move. 'Course, that's in theory; I haven't tried it before, but, I think it can be pretty cool. p.s. give Cstform a sandstorm forme. :(
