I'll be honest, I've been thinking about Yavimaya Dryad for a long time. It's like a Wood Elves, but you can totally seed some bad stuff for an opponent.
She also works with shocks and duals; you can donate a dual land and get other landwalks in for multicoloured.
Mirri is as viable as any other voltron commander. She's not my first choice, but she's aggressively costed, and there are a few ways to get forests into your opponent's hands (Gaea's Liege, Deepwood Elder).