Specifics: Instead of calculating the points lost by each loser as 20 * loser's score / winner's score, calculate it as 20 * loser's score / (average of all opponents' scores). The points gained by the winner would be the sum of all losers' scores as usual. Doubles games should continue to average by team.
Why it is needed: Under the current system, you lose fewer points if the eventual winner is a high-rated player than if the winner is a low-rated player. This causes a small incentive, in a situation where you have no chance of winning, to try to throw the game to the highest-ranked player. Also, as a simple fairness issue, the difficulty of a game (and thus, how many points you should be penalized for losing it) depends on all of the opponents in the game, not just whoever ends up winning.
The change would have an effect on rankings: high-rated players who play groups of lower-ranked players will get more points for a win, since each other player would lose more points, and lower-ranked players who beat groups of high-ranked players will get fewer points. I'm not sure whether this is desirable overall, although as a 2000+ player who plays mostly public games, I kind of like the idea.

Links to other suggestions: If this change is implemented, it would make it easier to implement Calculate Points At Time Of Elimination if desired.