In the spirit of the best chat client, MS Comic Chat. It would be nice to assign expression vismes to a wheel that could be accessed on the fly without the need for hand gestures.
I think the operation would be simple. Hold down a button and use one of the sticks/touchpads to place the dot on the wheel.
Further ideas could be multiple levels to each expression for varied vismes, should avatar creators want them. One downside to the wheel is you can only blend one expression with one next to it.