Rivalries
 

I'd like to see the ability to assign up to 3 'rivals' for each team in the team setup/options page - and preferably the ability to prioritize 1-3 (largest rival, 2nd most important, etc.).

The Rivalry games could/should have a couple of effects:
Increase/Decrease owner patience/manager firing;
Increase/Decrease player morale more
Higher attendance/$$ gate than normal; increase fan interest
Increase likelihood of player brawls/fights/game ejections

Spotlight the rivalry games in the league newspaper

I agree.

NYY vs. NYM should really feel like it. All other sports management sims I know have a rivalry mechanic.

What changes to the game outcomes does a rivalry mechanic make?

I like the idea overall, #1 (owner patience/firing is a little iffy) but in reference to Rich's post I would not want it to affect game outcome, though the OP did not have that in his list.

just my 2 cents

I would love this option. It shouldn't affect owner patience and firings but it definitely should affect fan interest, as far as attendance.