I think that having a Ron Paul / Alan Greenspan debate would be quite interesting. In the 90's Congressional exchanges between Paul and Greenspan, it was clear that Paul foresaw the dot-com bubble two to three years before it happened. Now that we find the housing bubble foisted upon us, it would be a fantastic idea to expose more people to the idea that the Federal Reserve is responsible for causing these boom and bust cycles.
If such a debate were conducted however, it may put Greenspan in a position where he would not endorse Paul. Given Greenspan's anti-Fed and Randian roots, I could see an endorsement of Paul as a definite possibility. The real question would be which is better, a Ron Paul endorsement or a debate? If I could write a letter to Greenspan, I would probably lean towards the endorsement request, as long as Greenspan can successfully ward off all the recent bad press regarding his ARM quote. I think Greenspan still has quite a following, and an endorsement of Paul would be even more interesting to see than a debate.