One of the things about Knockback is that you can just swing your sword and knock mobs back. That makes little sense and it does not really make good gameplay. I propose that swinging a sword (which will have a 0.3-ish seconds cooldown) should stagger the player or mob it hits, placing their attack on a 0.6 second cooldown. However, during that time, the player can still block. Blocking removes the stagger penalty from being hit, and causes the attacker's attack to be on a 0.5 second cooldown, during which he can counterattack.
To compensate for these chances, melee mobs (Zombies, Pigmen), like Skeletons, will actually swing whatever they use as a weapon instead of simply dealing contact damage (Slimes/Magma Cubes will be only damage a player if they are jumping). They will also have animations (obviously). In addition to that, pigmen and armed zombies will now block and zombies will have their attack range increased to 1.5 blocks.
Running and hitting or hitting stuff with a Knockback sword will still cause Knockback as well as lengthen the stagger time by a bit.
Stamina will be a hidden meter, much like Saturation. Having lower stamina causes you to do everything slower. Yep. EVERYTHING. From walking to running to fighting to mining. If you have no stamina, you will not be able to move or do anything that is not done instantly (right clicking on a furnace, etc). Your stamina cap and regeneration is dependent on your health. As a rule of thumb, Max Stamina = 80 - (Max Health - Health) * 4. You lose stamina by sprinting and attacking, and you gain it by not doing anything. As a rule of thumb, your stamina gain per second will be (Current Stamina / 2), rounded up. You can only gain stamina if you are standing still. Tasks less labour-intensive (like mining) will not consume stamina.
First off, sprinting will be buffed: you will be able to move up to 3X your normal speed when you sprint.
When sprinting, you will start out at your normal movement speed and slowly accelerate your movement speed to 6X the more you hold the control (or whatever your sprint key is) button. Double-tapping control or W will cause you to start at 2x your normal movement speed. Your hunger will only start to drain faster you hit 2X your normal movement speed, and it will drain faster the faster you move.
The distance mobs will be knocked back by your sword will be dependent on how fast you will sprint and how strong the knockback enchantment on your sword is. The mechanics will be explained in the next section.
You know how you can deal critical strikes by jumping before hitting stuff? Logic dictates that the higher kinetic energy gained by falling causes you to deal more damage. Do you know why you get hurt when you fall? It has a similar reason: velocity. In this section I would like to propose a few things: something akin to assassinating someone from above with a
hidden blade sword and transferring velocity to another object.
No, this is not purely an 'OMG ASSASSIN'S CREED MECHANICS OMG' section. This will also be a 'couch lance' section and a 'shove people into a pit of lava' section.
First off, you should be able to hold your blade in front of you by holding the left key. If you run into someone this way (even better: run into someone with a horse this way), you will sacrifice knockback power for more velocity damage. This will be a mechanic for wannabe Altairs and knights as well as users relying on speed.
Now on to the mechanics: running into a wall at a high speed will hurt yourself based on your velocity towards the wall (it will also cause the particles caused by falling, albeit in a different direction). This will apply to falling too, replacing the current (buggy) falling mechanics (remember the pig-on-a-leash where you cut the leash and the pig dies due to fall damage with the ground 1m away from him?) we have now. Yep. Your fall damage will be entirely dependent on how fast you are going before you hit the ground.
Here's another mechanic: kinetic energy transfer (also known as softening your fall by landing on a civilian in Assassin's Creed). If you fall on a mob (or run into a mob), you will transfer a percentage of your velocity to the mob. The velocity taken by the mob is then amplified by how strong the Knockback enchantment of your sword is, if you touch the mob by hitting them. This will cause the mob to fly away from you, possibly hitting a wall or another object where he will take damage. After you transfer your velocity to the mob, both you and the mob takes damage based on your velocities. This effect is amplified on the mob by whether you are holding your sword in front of you and whether you are pointing your sword at the mob.
No, I do not believe we need a dedicated physics engine for this. All we really need is to use the velocity system (Motion in NBT tags) Minecraft has to achieve the desired effect.