Do you foresee adding first-class language support for immutable object variables? Something like Java's "final" for member variables. I understand that something could be done with a @decorator to implement this, but I'm interested in baked-in support.

Fwiw, Adobe already /has/ open-sourced Flex, the Flash framework that really makes Flash useful for developing RIAs (they haven't open-sourced their compiler, I don't believe, but all of the Flex ActionScript is available). I'm a Flash/Flex developer, and at least a few times a week I grep through their source code to figure out how to do something, or how to change something about a built-in component, etc. Adobe has also released a specification for their swf file format, available at http://www.adobe.com/devnet/swf/.