I have been thinking about a few changes I think Mojang should make to mobs, as well as new mobs that could be added. I separated each into spoilers so it's more organized.
Silverfish Drops
This is something I have been thinking about for a while. I think that they should drop items called Silverfish Eggs. Now I don't mean spawn eggs, and that's not what these would do either. What you would do with Silverfish Eggs is craft them with cobble, stone, and stone bricks. You would craft them similar to how you craft Stained Clay, with the eggs being the dye and the block being the output, which I assume you already know. Using this recipe, you could craft Silverfish blocks. These would be useful for making traps in SMP Survival, Storing XP, and many other things.
Useful Charged Creepers
I think Charged Creepers, being the very rare yet highly lethal mobs they are, should drop a special, unique item. I was thinking something like "Electrified Dust" which could be used to make something special, or just be special altogether. I honestly cannot come up with an item they should drop, but I think it would at least make encountering one more productive.
Slimeball Potions
You can make potions with magma cream, which is made from blaze powder and slimeballs. You can also make potions from blaze powder. Why not be able to make potions from slimeballs? They could be used to brew mining fatigue potions, and if brewed with a fermented spider eye, haste potions.(A little off-topic, but I also think poisonous potatoes should be able to be brewed into potions of nausea, and fermented into potions of blindness)
Bane of Arthropods
You just enchanted you brand new diamond sword with 30 levels, and groan in anguish at your new Bane of Arthropods sword. Why do people hate Bane of Arthropods so much? The answer is simple- you don't encounter arthropod class mobs very often. If Mojang were to add more arthropod class mobs, Bane of Arthropods wouldn't be so useless. This used to be and sometimes still is a problem with Smite, but upon the addition of the Wither and Wither Skeletons, it has become a much more valuable enchant.
Ender Guardians
You have just traveled to the Ender prepared to defeat the Ender Dragon. As you begin to draw your bow to strike down your first Ender Crystal, you are knocked back by a powerful blow. You look back to see what did this to you, and see a frightening sight. Ender Guardians would be mobs that spawn around the Obsidian Pillars in the Ender. They would appear to be a set of 6 legs of a deep black color, much like all other Ender mobs. The legs would have an almost mechanical look to them. However, floating above the center of the legs is an Endercrystal. These mobs would guard the Obsidian Pillars, and would be the first Ender mob with a ranged attack. They would shoot ender pearls at you at a slow rate, much like the way skeletons worked before 1.5. The projectiles would fly like a blaze fireball, and deal a mild amount of damage but a large amount of knockback. Ender Guardians would run quickly, and circle around the player like a skeleton. They also have a visible charge up before their ranged attack like a blaze, where Ender particles will begin to condense around the Endercrystal head before firing the projectile. Also like blaze, Ender Guardians would have a melee attack if the player got too close to it while charging up an attack. For a blaze, this would use up the charge but deal a large amount of damage to the player. For an Ender Guardian, it is the same. The melee attack would not have knockback, however. I think this mob should be added because it would make the Ender Dragon fight much more difficult. For the final boss, the Ender Dragon is quite weak. All it does is ram into you and heal itself with explosive crystals you have to shoot. There are many enderman that pose a threat, but only to players that look at them. Imagine the fight with an auto-agro Ender Mob guarding each pillar and crystal. It would be quite a different battle.
More Golems
This is something that has been talked about before quite commonly. As of now, you can only make Snow and Iron Golems. What golems do you think should be added? Not just golems, either, any Utility Mobs. I think they should add one that shoots arrows, maybe with a dispenser head. It would look a bit silly, wobbling around with a bulky dispenser on it's head, but would be a ranged Utility Mob that actually does damage.
Nether Boss
Before they fully implemented the Wither, I thought it would be a boss you fought in large rooms in Nether Ruins. Though I enjoy the way it is found now, I still believe there should be a boss that spawns in Nether Ruins. What would it be? That's why we have to discuss it.
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"Minecraftia, a world of peace yet danger, a world of light yet darkness, a world of good yet evil. Every good thing you see has it's evil counterpart, evenly matching the power of the two. I intend, however, to simplify things down to one part, which just happens to be evil."- The Infector
Silverfish: Can't you use silk touch on these already?
Creepers: Although I agree that they should be more useful, I don't think they should drop a new item. Maybe just additional gunpowder.
Slimeballs: I agree with all the suggestions here.
Arthropods: Mojang already implemented three arthropods, so I don't think they need to add more, unless your Ender Guardian is how I'm picturing it (Like a massive spider, but with enderman limbs for legs), or there's a new golem that's kind of arthropody.
Golems: Perhaps. The dispenser golem would be interesting.
Boss: I don't think they should add more bosses until they clean up the existing ones. The Wither was originally supposed to be a Nether boss, so perhaps they could change it back to being only spawnable in the Nether (in survival mode).
Silverfish Drops - Alright. Ultra Charged Creepers - Don't know what to say, by the time I made this exact post, you didn't suggest much there. Slimeball Potions - The mining fatigue ones would be useless, but I like the nausea one. Bane of Arthropods - At least tell us what the new arthropod mobs would be. I don't we need more anyway. Ender Guardians - No... Please, no more Ender stuff. We've got enough "Ender" in the game and we're tired of those suggestions. More Golems - Err, nope. We have 4 utility mobs. No need to shove in more golems. Nether Boss - This forum is for YOU to make the suggestion, and not us.
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Hey, you there. Yes, you! Are you thinking about posting a suggestion on the forums anytime soon? If so, please read this before doing so.
Silverfish Drops
Useful Charged Creepers
Slimeball Potions
Bane of Arthropods
Ender Guardians
More Golems
Nether Boss
But the silverfish eggs seem pointless without multiplayer.
Play minecraft.
NOW
Creepers: Although I agree that they should be more useful, I don't think they should drop a new item. Maybe just additional gunpowder.
Slimeballs: I agree with all the suggestions here.
Arthropods: Mojang already implemented three arthropods, so I don't think they need to add more, unless your Ender Guardian is how I'm picturing it (Like a massive spider, but with enderman limbs for legs), or there's a new golem that's kind of arthropody.
Golems: Perhaps. The dispenser golem would be interesting.
Boss: I don't think they should add more bosses until they clean up the existing ones. The Wither was originally supposed to be a Nether boss, so perhaps they could change it back to being only spawnable in the Nether (in survival mode).
If you want to talk privately, click me...
Ultra Charged Creepers - Don't know what to say, by the time I made this exact post, you didn't suggest much there.
Slimeball Potions - The mining fatigue ones would be useless, but I like the nausea one.
Bane of Arthropods - At least tell us what the new arthropod mobs would be. I don't we need more anyway.
Ender Guardians - No... Please, no more Ender stuff. We've got enough "Ender" in the game and we're tired of those suggestions.
More Golems - Err, nope. We have 4 utility mobs. No need to shove in more golems.
Nether Boss - This forum is for YOU to make the suggestion, and not us.
Locking.