I never played UO and I've never spent a great deal of time playing any MMO. I know it would take a lot of effort but I think a neat solution would be that if all the animals were killed then people would start starving to death.

I wonder if social pressures could somehow be codified into a game. If someone goes around killing all the players then maybe none of the vendors will want to do business with that person or something? Though I'm sure there are ways players could still explain that and I'm sure people who have spent a lot more time thinking about MMOs than me could explain why that wouldn't work