I like this idea too.
But I think the tail emotes should just be additions to the regular emotes. Instead of just a face emote, the model goes through the whole thing--face, body(posture), and tail.
When I first played the game I thought 'butt swing' was supposed to be 'tail wag'...
... I'd much rather have 'tail wag', no offense.
But, speaking of that! I would like to have a 'default' tail setting. Up, down, idle wag (down and wagging), full wag (up and wagging).
For cats it could just be up, down, mid, and idle wag.