Here's my own tentative proposal (besides my variations on Buchanan's proposals). It may be even more important and interesting than Buchanan's (as important as those are); why is left as an exercise for the reader. I call it the Jurisdictional Choice Amendment.
"No law shall infringe an arbitration clause in a contract, nor shall original jurisdiction of said arbitrator over any area of civil law of the United States or any unit of government therein specified in said clause be denied. The courts of the United States and of any unit of government therein shall defer to the findings of fact of said arbitrator unless clearly erroneous."