How did I miss the typing discussions?
Grass/Ghost
The reason for this is simple. Do we really want a Dragon-type with access to Shell Smash?
Hell No. Even if it's secondary typing is Poision... which is actually a pretty decent defensive type with all of the fighting types running around OU, Dragons with access to Shell Smash, Gear Shift, Quiver Dance, Spore, and similar moves, especially when focused around Bulky Offense, is a massive no-no.
Grass/Ghost is far less likly to be broken by Sketch, but is also not useless. Not to mention it's a feasable type if you want to take a more defensive approach. Only real downside I can see is that Rotom-C is arguably already a Bulky-Offensive Grass type.