I really like the slate we have so far. The only problem I see with Illusion is that the only pokemon it can disguise itself as without revealing it's presence via stealth rock damage are also rock weak. So it doesn't really help with the glaring rock weakness. Other than that it is quite neat, would make a good lead.
Also known as Monkeybillio and RadicalMonkey
Check out my RMT: Shiny Rocks