The electoral college
This might well be preferable to the system we have now (and requires no constitutional amendment inasmuch as the "winner-take-all" system is the product of state decisionmaking, not of any constitutional requirement), BUT I have one huge reservation: We are already suffering terribly in the House of Representatives from the consequences of "partisan gerrymandering," where, as two friends of mine once wrote, "representatives pick their voters rather than the other way around." If we shifted to the district allocation mode of assigning electoral votes, there would be ever greater incentives for both of the major parties to engage in ever worse gerrymanders and the further poisoning of our politics. Thus, if we are to retain the College, I would prefer that votes simply be assigned on the basis of statewide percentages, with the two "Senate votes" going to the statewide winner. This would basically eliminate the "human dimension" of the electoral college.
I recently engaged in a debate on the electoral college with professors John McGinnis from Northwestern and Daniel Lowenstein from UCLA, both of whom vigorously disagree with me. You can find it at http://www.pennumbra.com/debates/debate.php?did=8
My own preference is to eliminate the electoral college and shift to a popular election with a mechanism to assure that the ultimate winner can plausibly claim majority support. The French have a two-stage election; San Francisco has an "instant run-off" so called Alternative Transferrable Vote. No doubt this would be one of the most vigorously debated issues at a Convention.