Thanks for sharing your ideas and now I can visualize what you want to create--very interesting...
"I imagine it would be cool to log on a website with a big bank of questions, navigate, try many questions and flag the ones I want then click on a button and download my choice of questions as an xml file"
Yes, that is what I would very much want to do too. The key would be making it very easy to browse, very easy to put those questions into your course, and very easy to donate questions. I think you are right person to make this happen--as a teacher, programmer and long-time contributor to Moodle. Let me know how I can help.
I also think that MoodleNet should include this. Not that MoodleNet should do it, but MoodleNet should include this "Questions Hub" in their collaborative teaching authoring environment.
I wonder if there would be any problems handling or viewing video, audio, images in the questions?