A mod that grows mobs from seed-like items. As the plant grows, it starts taking on the characteristics of its mob until the plant ripens and the mob is released. For example, killing animals would randomly drop corrupted flesh that can be planted to grow a zombie, bones grow skeletons, breaking leaf blocks randomly drop a sinister vine to grow a creeper. Passive mobs could be grown by combing carrots, barley or wheat with growth medium to yield pigs, sheep, and cows.
The mod could be used in situations where normal mob spawns are disabled but you need the resources for progression.
Had an idea like this a while back for Creepers due to there plant-like nature. The idea would be that creepers would explode and have a 5% chance to spread spores. The spores would then fly away randomly and take root on the ground. Then the creeper would grow over time resulting in a baby creeper. Which will then grow into a full creeper, potentially with random effects based on growth block.
If i ever get around to making the mod I will keep this idea in mind :)