I wrote a thread about this before, however, I have rethought the idea and made some changes to it.
Minecraft has no stealth mechanics, only a boring crouch, and invisibility potions. You cannot sneak up on a mob whatsoever, they always see you even if they are not looking at you. Where's the fun in that? I think we should be able to ninja our way past mobs, or perhaps even defeat them using stealth.
The detection radius's of all mobs should all be moved to only in front of their head model, meaning if they are looking away from you, they will not see you. Different mobs have different vision distances, with undead mobs having shorter distances and other mobs, particularly Illagers, having much further vision. Depending on the light level of where you are standing, you can be harder or easier to detect, with lower light levels making it harder for you to be seen, and higher ones making you easier to be seen. Pitch darkness completely hides you from a mobs vision, except for Spiders, Cave Spiders, and Endermen, as they appear to have natural night vision. However, other mobs can still hear you in darkness and can track you by that.
If the mobs have night vision, you are on fire, have a glowing effect, or you are wearing enchanted armor, this is completely negated, and they can see you regardless of the light level. If the mobs have blindness, it will drastically reduce their detection radius, and the player having a glowing effect results in mobs seeing them through walls. Also, the faster you are moving, the quicker you are detected by a mob. If a mob sees you, unless it is killed quickly, other mobs nearby will be attracted to attack you as well. Making it out of their detection radius will result in them continuing to go the direction they last saw you, and if they don't manage to see you again, they will stop giving chase.
Obviously, bumping into a mob will also make them immediately detect you. Hitting a mob with an arrow or throwable will result in them approaching the location it came from, searching for you.
Mobs can hear any sort of action that produces sound, particularly footsteps, which they can hear from an 8 block radius if you are walking, and 12 if you are sprinting. Walking on some blocks can affect this range, with grass and sand reducing the range. Crouch walking or crawling renders you completely silent, unless you are sprint jumping while crouched, which is equal to walking. Mobs will search out any unusual sounds they hear, this includes things like arrows, splash potions, and snowballs, and they will immediately start to approach where they heard the sound. Full blocks between a sound and the mob will muffle the sounds and thus make it harder for the mob to hear it.
Falling from heights greater than a block and a half will also make sound, with the sound being louder depending on how high you fell from. This can be suppressed by crouching right when you hit the ground, unless you took damage from the fall, to which it will only be partially suppressed. The sound of opening doors, trapdoors, chests and barrels will also attract mobs, which can be suppressed by opening them while crouched, but it will take 3 seconds before it actually opens in this case, and looking away from it before it opens will cancel it. A new enchant known as "Silent Step" can be applied to boots, which reduces the sound of your footsteps for each level, which goes up to 5.
All variants of Zombies have a unique way of detecting the player, they can sniff them out, which can even allow them to find a player who is invisible or in pitch darkness. This works by a player staying near a Zombie for too long, to which they will begin to smell the player, and start to approach where they are at, searching for the player. They can also sense when a nearby player takes damage from being attacked by a player, or another mob, which also results in them approaching your location.
Players can perform a "stealth strike" on any mob, by critical attacking a mob that is unaware of their presence. This attack will greatly multiply the damage dealt, instantly killing mobs that are in similar size to the player, depending on the weapon used, while bigger mobs will not receive as much damage from a stealth strike. Neutral mobs can only be stealth struck if they haven't yet seen the player in their neutral state. If a stealth strike does not kill the mob, they receive a great deal of knockback, depending on the size of the mob, which can potentially be used to push them into traps, lava, or off cliffs. If a mob is killed by a stealth strike, they will make no death sound, resulting in the player staying hidden.
You can only stealth strike a mob that is attacking another entity if that mob hasn't heard or seen you yet. Stealth striking a mob will grant the player an advancement called "Ninja."
What do you guys think? Like the idea? Think it needs changes? Or have any questions? Let me know.