I think the main stat to worry about is speed. If a pokemon is able to learn any ONE move, it should be able to surprise the opponent with that move quick and fast. I also agree that its movepool outside of sketch itself should be small. Otherwise, it could increase the power of duel screens hyper offense or even (Gasp) the dreaded baton pass teams. We do not want a carbon copy of smashpass smeargle. However, its main roll, I believe should be support. If it has too strong offenses, that one move could easily be something like sacred fire/ V-create could make it less fun. Two moves that I would want to see in this pokemons movepool are substitute and rapid spin, to get a heads start. Im seeing this pokemon to be something like gengar, capable of running an offensive set, but is used more as an annoyer set with substitute.