This is a good point. Thinking up all the rules I did for the last Cup was a lot harder than I expected it to be.
Of course, there's no reason we always have to have three rules per round. If the moderator gave one thematic rule (i.e. "Make a comic about Christmas", as opposed to "use Diablo on the right side of panel 2"), that could work out just as well. But then, like skagg said in another thread, part of the idea of throwing three random rules together is to see who can make the best strip under adverse circumstances. What I'm saying is, I like the way we've been doing the rules so far, but if another moderator wants to do it differently, that's certainly an option.
At the risk of overstating the obvious, I want to point out that whatever methods we use in the Cup should get the most people the most playing time with the shortest lapse in between. The less time between Cups, the less upset people will be about not making it into one. That's a strong argument for limiting the brackets to 8 contestants.
In the same vein, I like the idea of a 24-hour time limit on voting. If the moderator always reserves his/her vote until it's needed, then it can be used to break a tie after 24 hours, or not used at all if there isn't a tie. That should keep things rolling along smoothly.
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The what mentioned above is total fiction. Please don't take it seriously!