Rivalries, which would have the effect of driving up attendance and, possibly, slightly tweaking player performance (a la the "clutch" rating). To wit:
1) Geographical - the game should be aware of how close two cities are to each other and increase fan interest accordingly.
2) Evolving - if two teams are historically battling for the pennant, or have played an inordinate number of close games, rivalries can evolve naturally.
3) User-selected, so you can "force" a rivalry if you have a particular storyline or some other reason.
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