I would love to see the craftable mob spawner blocks that are in some other games. They activate and spawn mobs when a player is near. Mobs that have spawners occasionally drop eggs when killed which is a placeable mob, and the egg can be used to craft the spawner. I know that the eggs already exist for placing the mobs, so it would be a matter of making each mob occasionally drop its egg when killed, and implementing the craftable blocks.
Spawners could contain animals such as chickens, or cows, or they could hold more sinister things like oerkkis, uruk-hai, mummies, alligators, even balrog, though perhaps that drops its egg only one time out of a hundred or less.