The current UI components are all XAML based. Flash probably won't even drop in the consideration box.
I don't expect this to change unless they redo the entire client.

As it is in the current state - Highly customizable, just that SE has it encrypted and has no editor for people to use.

If you have never heard of XAML, you can go run a search for it. (Related tech: WPF/Silverlight)