I think the hidden "aggressiveness" part of each player is enough for us to worry about. And also, I don't think it's really in the spirit of the game to encourage teams to have a "play dirty" option. Just thinking about what could happen conjures all sorts of crazy thoughts in my head. (And I'm not just being paranoid that everybody will always save that dirty game for my team, though that idea does come to mind...)
But, just thinking constructively here since I do not find the OP without merit, maybe it's worth discussing the possibility that Rival games should maybe have a slightly increased number of fouls. To go with this idea, we'd have to first agree that Rival games do indeed translate into more fouls, say in real life (somebody want to do some research into this?). Second, we'd have to figure out what to do about the "Rival imbalances" since it's not uncommon for some teams to to be assigned the Rival for more than 2 other teams (I seem to recall one team being assigned the Rival of 5 other teams once, but I can't remember where I saw that) while many teams are nobody's Rival. It really wouldn't be fair to increase the risk of injury from fouls more for some teams than others.
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