I'm not sure if any ability could specifically this suit this Pokémon, to be exact. In its own way, Smeargle is pretty special. Its offensive stats are not good, and neither its health or defenses, but it could learn any move out there that exist in-game, and it makes this Pokémon stand out by hundreds of miles. Due to that, it's usually utilized as a support, so that you could gain stats boost and pass it onto your teammates, set up entry hazards, and so forth.
They've tried giving it Moody before, but it didn't work out too well. The ability would lower one of Smeargle's stat at random, and sharply raise another, if I'm not mistaken, which was considered to be sort of broken. Due to that, it was banned from competitive play. If it winds up getting something such as Intimidate, I suppose it could be useful in some regard, like when going up against physical attackers. What I would personally like to see, however, is for it to somehow gain Magic Bounce. This one would be quite helpful, if you ask me. Most of the time, strategies that we tend to use with Smeargle--which involves using non-damaging moves--get crippled pretty easily when foes use Taunt. If it had Magic Bounce, that could easily be avoided, I believe, and we'd finally be able to carry out our plans thoroughly without any interference.
Also, this thread would fit better in Pokémon Gaming Central as it's kind of a broad topic, so if you don't mind, I'm moving it there for you!
Moved.