Currently I'm working on compositing/window decorations based on the ideas of the metacity/gtk-window-decorator. Afterwards I want to make something similar to eg. the Qt layouting system with drawing elements similar to most common GUI toolkits like for example GTK. They make heavy use of images which can be scaled using 9-scale/image slice. So for default dialog elements like tabs/buttons/etc. we can probably just reuse existing (eg. GTK) themes (at least the images) but also create a special FlightGear theme. What probably is more interesting is to create dialogs/layouts unique to flight simulators, for example fuel and payload (will probably be different for each aircraft - at least a 2d drawing of the aircraft layout) or a weather dialog for showing and changing the current weather, or a layout for the virtual checklist dialog, the aircraft help/tutorial/etc. dialogs or some icons for the menu. You probably already got the idea
I think creating icons and layouts for dialogs is much more useful than creating common gui elements with many existing (also GPL licensed) images.