Variants I'd like to see:

1) You can observe rounds played by other bots.

2) You can partially observe rounds played by other bots.

3) (The really interesting one.) You get a copy of the other bot's source code and are allowed to analyze it. All bots have 10,000 instructions per turn, and if you run out of time the round is negated (both players score 0 points). There is a standard function for spending X instructions evaluating a piece of quoted code, and if the evaled code tries to eval code, it asks the outer eval-ing function whether it should simulate faithfully or return a particular value. (This enables you to say, "Simulate my opponent, and if it tries to simulate me, see what it will do if it simulates me outputting Cooperate.")