Hello everyone! I am a reditter, I post the most on /r/minecraftsuggestions thanks to the good ideas I gather, so I thought I'd share some with PMC. I will list these in order from most to least popular.

1) More achievements due to all the new features since MC 1.0
Includes achievements for mining emeralds, trading, taming a wolf, brewing a splash potion, smelting gold, crafting armour...

2) /strike or /strike command, allowing a player to summon lightning at specific coords, or targeting a specific player

3) As the wither loses health, it loses a head for every 1/3 of health, when its down to 1 head, it will no longer be able to fire and then start charging at you

4) New weather type: Fog, decreasing render distance automatically, causing everything to appear misty and making the sky beyond sight, thus making fighting tricky

5) Unlocking an achievement gives you xp, the amount varies on the difficulty of the achievement

6) Ability to customise Skeleton, Wither, Witch, Ghast, Blaze and Snow Golem firing rates using NBT tags

7) New firework effects: Multiply (divides into 5 separate rockets, additional effect), Dome (creates a small dome shape), Large Dome (obvious), Flame (flame shaped), Long-lasting (particles last until they come into contact with a solid block, additional effect)

8) Enderman improvements: Endermen have a 3 second 'cooldown' after teleporting, they will drop the block they are holding when killed, attack animation similar to zombies when holding a block, damaged when picking up cactus

9) Add dead forests to the Nether, made of new Netherwood type, dont have leaves, dead bushes can spawn in this biome, netherwood burns forever like Netherrack

10) A mob will only attack/notice a player if it is facing him

11) New flower: the Lily, can be planted on lilypads or in flower pots (lily pads will also spawn in jungles), 10% chance of spawning on a lily pad

12) Customize option in creative inventory, allows you to add custom blocks/items from your inventory (as well as edited books, dyed armour, /give items...), move around blocks, move around sections, change 'representative items', adding more empty slots

13) Mushroom Island M technical biome with much a more packed population of huge mushrooms

14) More river features, includes meanders, ox-bow lakes, waterfalls, river cliffs...