Varying Mob Strength

This could be a controversial change, but aggressive mobs such as the Zombie and others have become too easy. You know with a stone sword you can kill a Skeleton in 4 hits, and 3 hits with a Diamond one. It would be so much more exciting if they could spawn with maybe 15-30HP with increasing rarity for higher HP. Maybe they could signify a larger health pool by having deeper mob sounds. A deep groan from a zombie could hint at a strong zombie. Then you do not know how fast you can kill a mob keeping combat fresh and exciting.