I like the idea of Gray as a "special" color, and there's actually a good reason to keep it that way. If you include Gray as one of the colors in a Treehouse set, you won't be able to use Volcano Caps with that set to play Volcano (or any other Volcano Cap game).
For these reasons, we've already been planning to keep Gray out as the single monochrome stash that will survive the revolution.
I think Brown would be a great opaque color.
That's what we've been talking about lately too. The set needs to include a solitary opaque, and Chocolate sounds good to me.
I also like the idea of "Smoke" as a translucent color.
This and something in the magenta/hot pink area are strong contenders.