It seems strange to me that DLC always takes the form of permanent content... always as objects, rather than events.
In the case of MMOs, it would be hard to charge for events after so many years of including that service in the subscription package. But I'm surprised games like Call of Duty 4 and Rock Band 2 don't have pay-to-play events.
It could take any number of forms. Tournaments could offer real prizes, which were bought with entry money. Music games could offer songs as temporary downloads (surely, record labels would make more songs available then). This stuff could be offered for pennies.