I've added spawning snowballs. String spawns spiders for instance, but add glowstone and you get cave spiders. Skulls have special handling so we can spawn powered creepers, villagers zombies, etc when amped up with glowstone.

We spawn cows for milk buckets, or mooshrooms if amped, endermen for eyes of ender, ghasts for ghast tears, and skeletons for skeleton skulls.

For wither skelly skulls, we spawn wither skellies. But if amped by two glowstone blocks, we spawn a wither. Take that!