Maybe you could reduce the effect of kingmaker by using the effect of kingmaker to reduce the distance between players. Something like catching crooks with crooks:). Let's say one player has 2 points another 4 and the third player has 6 points. By using an bidding system (maybe you could also make other applications) and making the player with 4 points the bidding master he could choose between giving the player with 2 point cheap commodities or whatever or let the player with 6 points pay a lot of money for the goods. In order to implant this there must be some mechanism wich says that the second highest player must be the bidding master. i don't know or it would work but maybe.
I had the same idea, sort of... I think what Trasa is hinting at is adding kingbashing to the game mechanics themselves, because as I read these posts, (many good ideas btw) I seemed to see that keeping the game close is key to avoiding kingmaking. I was inspired with the idea of having "tiered" scoring. What this basically means is that different players get different point values for the same actions. Now before you say, "But what about when the leader is clearly a more skilled player? Wouldn't this just be a way to benefit poor players for their poor playing?" Trust me, I am 100% against giving incentives for poor play. However consider this example of what I mean.
In the trading game "traders" we'll call it, the basic way to score is buy and sell commodities. However, a rudimentary "economic" structure exists so that players with strong economies have higher prices for the goods they sell. At the same time, weaker economies sell the same goods for lower prices. This encourages players to buy from weaker players, thus making them stronger. It also brings the stronger player's prices down, as their demand drops.(To calulate simply, "if you have the most points out of everyone during the trade you get 1 pt, 2nd most pts gets 2 pts...least points gets 5 pts...etc") To curb the negative, give the stronger players the ability to invest in goods that are always high priced, but expensive initially, that way a "better" player won't simply find his/her hands tied until dropping to a lower rank, but will also be unable to leave others completely in the dust. You can also have a "shop" where all players must go to gain items that help them. The greedy game will ask a higher price of wealthier clientele. (The way U. S. income taxes would work if rich people weren't allowed to evade them...) This will help to balance out the overall available funds. At the same time however, side-missions a-b-c etc. exist, without tiered scoring, so while they are more difficult to get, the stronger you are, the better your chances of being successful. These "un-tiered" missions will be weighted to be more important to winning, so that a skilled player is checked by the game, not punished, as this would essentially just move the role of kingmaker from losing players to the game itself. You could even go a step further and restrict "helping" in the game by making trading with the same people less profitable over time. (1st time 5 pts, 2nd time 3 pts...etc)
Granted, this entire game is really just a very rough sketch of some possible solutions by making it harder to advance the better you play, but hey, I figured that since I've seen what kingmaking can take away from a game, I thought I'd take a stab. I dunno, maybe none of these ideas would work at all, but hey, this IS a public forum right? :^"
I think it's important to make a distinction between having an interactive game, and kingmaking. Of course, in an interactive game there will be moves that decide the future winner. However, players will make these moves because they believe it will improve their own chance of winning the game, or at the very least their standing in the game, even if in the end it turns out that it didn't. So, this is not really "kingmaking" in my book.