Take a look at Lilligant. Also has grass typing, also is neutral to Rocks, also gets Quiver, but has a terrible movepool and therefore isn't used much. Its stats also hold it down.
I don't want Sketchy to end up like Lilligant.
The thing holding it up above Lilligant is the fact that it has a good movepool with its Ghost stabs and a coverage move. However, I'll agree that it should have something above Lilli and Volcy in terms of stats, because just switching in more and having a different set of coverage moves won't really be that spectacular of a change.
Physical sweepiness: Very good. This will give it the ability to hit hard with somewhat weak ghost stabs like Shadow Claw or whatever, and make use of Dragons Dance, Agility or Swords Dance. Also gives possibility of Choice Band or physical Scarf set, especially the latter if its ghostly nature gives it trick.
Physical tankiness: Good. Tanking hits should be pretty important for it, and it shouldn't need too much of this to take a fighting-, normal-, or ground-type attack, or a waterfall for those rain teams.
Special sweepiness: Good. Everyone's going on about Quiver Dance and stuff, which will also give it the advantage of a Shadow Ball or especially Giga Drain being boosted. We also shouldn't overlook a Nasty Plot set, which will make things like Focus Blast if we choose to have it work well. Also, Seed Flare getting sketched gives it a killer arsenal for a mixed set.
Special Tankiness: Great/Excellent/Whatever we call above Very Good. Yes, that is the full technical term. Everyone agrees that seeing its counters and its resistances will make it want to be heavily invested in special tankiness, especially since boosting moves outside the more offensively-oriented Quiver Dance.
The difference here between Lilligant and Volcarona is (A) the ability to go offensively mixed, and (B) the idea of not having completely shitty stats. Due to sketch, it should certainly be able to pull off both offensive and defensive roles, so given these maximums, it shouldn't be too hard to pull off the bulky offensive, purely defensive, purely offensive, or powerful defensive roles with a generally rounded stat base.