They're a bunch of mods out there that add massive, randomly-generated structures to Minecraft. They're mods that add castles, fortresses, cities, etc. But in vanilla Minecraft, the only structures you can come upon are jungle/desert temples, dungeons, villages, abandoned mine shafts, and strongholds. All these structures are relatively small o mine shafts and strong holds can get pretty large, but they don't appear above-ground, and they lack the aesthetic appeal that jungle/desert temples.

There's something about rounding a corner and seeing a temple appear before your eyes that can't be matched by stumbling upon a labyrinthine passageway underground. But then again, the size of these temples leaves a bit to be desired. Imagine this:

You're hiking through the mountains, you're running out of stamina, you're lacking the equipment needed to survive through night, and the sun is just about to set. You're getting pretty desperate to find some sort of shelter o a cave, a hole, anything to hide you from the monsters that lurk through the darkness. Then, when all hope seems lost, you reach the top of a peak and are presented with the sight of a beautiful valley stretching before you. In the valley sits a massive castle, complete with a moat, drawbridge, multiple towering guard towers, and a huge central keep topped with a giant spire stretching up into the clouds. The castle is easily a many hundred blocks around, and it practically takes your breath away upon laying eyes on it.

That is my suggestion for a future update to Minecraft: Castles and/or Other Massive Structures. If Mojang could get a few skilled builders to team up, they could easily create a beautiful medieval castle or fort. They could even create multiple, biome-specific variants of the castles to appear all over Minecraft. They would be extremely rare structures, but the feeling you would get at finding one rival that of loading up a brand new world for the first time. There could even be villagers inside the castle. Different types of NPCs could be introduced, like guards, kings, and queens.

I think that if this idea is incorporated into vanilla Minecraft, the game would improve so much. This idea is completely my own, though there are probably some similar suggestions out there. I take full credit for this contest entry, though the castle picture included is one I found off of Google, and I don't know who built it.