Now, I'm not going to reiterate what people have already said, but I think these ideas are origional. This post will be broken into three parts. Consumables, Tools/Weapons, and Mobs. Click on the fallowing links to switch between the posts.
Potion of Frenzy: all damage dealt are critical hits for 30 seconds. Created with the base potion being a potion of streingth, and the ingridient being Rose Red
Potion of Cursing: Created with the potion of Frenzy as the base and fermented spider eyes as the ingredient. Applies the "Cursed" debuff.
Potion of Ice: Crafted with the Akward Base and a Snowball. Applies the "Frozen" debuff. If a splash potion, the blocks around will be covered in snow, or if it hits water/lava, it will freeze/turn into obsidian.
Alchemists Flare: The potion that sets you on fire if dranken, or in the splash form, sets every block affected on fire. Created with an akward base, and netherrack as the ingredient.
Gelatin: This I just hope is added because I'm SOOO easily ammused. Basicly, its a placable block like cake, that can be eaten 6 times to heal 1/2 a hunger point, but it jiggles when you touch/hit it. :3 I think it would be fun. It's crafted like so:
Feather=Melon/Sugar (interchangable) Teal=Bonemeal Diamond=Snowball(damn it, when are they going to update the emoticons?)
Placable like Cake, but different textures. Can only be eaten 3 times, but for 3 hunger points each. Crafted like a cake, but with 3 wheat on top instead of milk, and an apple/pumpkin in the middle instead of an egg.
Carrots: There are no vegitables in the game, and I like carrots, so carrots it is. In the wild, there will occasionally be a white flower, when hit, they will drop carrot seeds. The carrots will grow like mushrooms once placed, but only on to farmland. The carrots can be used to tame horses, and make beef stew.
Beef stew: Just like Mushroom Stew, but with a carrot and beef
Mead: A brew you make at the brewing stand (bottled water as base, wheat as ingredient). It heals 3 hunger points. The first one you drink is okay. Once you drink the second one, the screen begins wobbling back and fowarth. Drink the third one, and you have food poisoning, the screen is completley disorienting, and you begin seeing 3 of every mob. It would be fun to run around. I mean, tell me one game with an alchahol option that you never abused?
Cider: Prepared same as mead, but with an apple. Heals all 10 hunger points.
Calendar: A placable block crafted like a compass, but with a clock in the middle. It tells how many minecraft days it's been since the world was created, and the phase of the moon for the fallowing night.
Spyglass: A tool used to zoom in far to locations that are either beyond normal rednder distance, or to see mobs that arn't visible yet. The most prominent use would be to scout out an area from a high vantage point, safe from mobs. Crafted like so:
Crossbow: A bow that works like a bow before 1.8. It has a severley de-buffed firing rate, though, with every shout requiring a 1-2 second long reload animation. Drop from dungeons, but about as common as a saddle.
Scoped Crossbow: When you place a crossbow under a spyglass in the crafting menu, you get a Scoped Crossbow. This weapon has a significantly longer trail before dropping off than both the bow and crossbow. When you punch, you zoom in, and right-click to fire. Slightly longer re-load animation as the crossbow, so not that useful in close-quarters. Prime function is to clear out an area before advancing.
Repeating-crossbow: Crafted when you put a redstone repeater on top of a crossbow. This crossbow is silver, and will fire 15 rapid shots, each dealing 1/2 a heart of damage. It isn't supposed to be used for killing mobs, just spraying fire into a crowd to weaken them to about 5 hearts so they can be more easily killed.
Map Markers: Just an add-on to the map. If you right-click while holding a map, it allows you to draw on it. It has 15 pre-made stamps (each represented by the colours of wool) that you can drag onto the map to mark points of interest. You can also draw on it, and make a legend.
Weapon Enchantments: I'm catigorising these under weapons for sake of convienience.
Enchantable Bows: Bows can have all the enchantments a sword can have, it just costs more levels.
Magic feats: a special aspect that can be activated by right-clicking twice with your weapon. These will be labeled as MS.
Frost Tounge: Freezes enimies
Spider Bite: Poisons enimies
Sacrificial Blade: When you kill an undead with this weapon, for 1 level, you get to bring it back to life to do your bidding. To avoid this, don't kill undead with this blade.
(MF)Druadic Blade: When right-clicking an animal, they will fallow you, and attack anything you attack. They will only do 1/2 a heart of damage, but are loyal and will fallow you to the death. They can be told to sit like wolves.
(MF)Ghasts Wrath: When right-clicking, it will fire a ghast fireball at enemies. Costs 2 food points (as summoning raw power from yourself does make you hungry)
(MF)Blazes Wrath: When right-clicking, for 1 hunger point 3 blaze fireballs will be fired at enemies.
Your Eternal Reward: You gain double exp for everything you kill with this weapon!
How Shocking: Stuns enemies
(MF)Thundering: A weapon that when right-clicked, will strike lightning onto the place that was selected if above ground, underground it just strikes lightning on the same XZ coordiantes, just with a Y coordinate that is closest to the sky limet, if that makes sense.
Now, we all know and love our current debuffs (On fire, Poisoned, food poisoned, and the varius negative potions), but here are a few more that change the gameplay.
Cursed: End beings will curse you. When cursed, all damage taken for the next 30 seconds will be a critical hit. This makes fighting endermen even more challenging.
Frozen: When in a snowy biome durring a thunderstorm, you can get frozen for being on a snow or ice block for to long. When frozen you have max defence, but can not move.
Stunned: After being shot by lightning, or shocked in general, your movment becomes twitchy and switches the controls form moving normally, moving in the oppisite direction, and jerking from side to side every second or so. When you get in water, it electrifies the water, whick will stun mobs other than a creeper within a 15 block radius if in the water. The creeper would just be supercharged.
Short-Circuting- If you flip a switch on and off a whole bunch of times, or lightning strikes a power source, the redstone wires begin flickering on and off, shocking (stunnning and doing damage to) anything nearby, before ultimatley turning all the redstone affected by the charge into thier dust form.
Horses: I'm not going to go on about this, because the Mo Creatures mod does everything there is to do with horses right. Now if only we could get that into vanilla minecraft...
Interactable NPC's: NPC's will begin to attack you if you destroy their homes, if you attack them, they call the gaurds to attack you, and all hell breaks loose. On the other hand, you can trade your things with the respectable dealer and get profit off of it.
Interactible Zombie-Pigmen: Gaining the zombie-pigmens trust and gold through giving them water and food. If you give them enough, you can give them weapons, and they will fight for you.
Pigmen: Remember when pigmen were going to be the NPC? I miss those days, too. Maybe someday notch will add them into the game.
Well, im out of ideas. I gave the ones that were taken from people the link to the forum post, so you can't say i didn't give them credit. And if someone posted one of the ideas before me, I hope you will think that it's possible that two people had similar ideas, but if you must, go ahead and point me to a link to that forum post. I'm open to critisisms, and people posting tl;dr will get reported.