So ... here's the overview of the project.
The green box contains the most basic possible release of the software - no real whistles and bells beyond getting something useful that we can start using / testing.
The boxes above that are the language / saved file format stuff.
The blue boxes are things for versions 2 to 100 ...
All saved data / parameters / language specs etc will be in xml. The entire app will be written in AS3 only - nothing specific to Flex or Flash. No assets ... well, maybe a png library later, but it should be possible to compile the project from either an empty FLA or a simple Flex project without requiring any assets.
So - with the whole project written in pure AS3, anyone who uses the software will also be able to modify / adapt / enhance / reskin it.
In terms of UML elements, I think we should start with the most commonly used elements and then add further ones later.
As far as interface / usability goes, my 2 favourite pieces of software are coda, by panic, and omnigraffle by omnigroup. If we get anywhere near to something as beautifully usable as those two I'll be a very very happy bunny.
The 'class swatches' add on is something I'm kind of excited about myself. It would allow you to have templates for classes / design patterns that you drop in and edit.
I'll expand more on the diagram above as soon as I get a chance, and it's by no means a concrete solution, just the one that I've come to so far - all feedback / ideas gratefully received.