Minecraft: New mobs and improvements to the Stronghold and the Ender Dragon

Bear and Leopard: neutral mobs to make the world feel more lived in. the latter would inhabit forested and arid biomes, and would have Black and Snow variants.

Troll: appearance-wise, it's the Iron Golem - but flesh and blood, stone grey skin, and yellow eyes. it'd inhabit wooded biomes (including the Pale Garden), mountain biomes, caves, and Strongholds. turns to stone in daylight.

Wyrm: appearance-wise, it's the Axolotl, but lacks gills, is grey-ish white, is 4 blocks long, and can open its mouth. it'd be as slow as the Iron Golem, but can turn around quickly. it attacks by spitting fireballs which can only burn mobs (and nothing else), cause minor knockback, and cannot grief terrain or be deflected, and a venomous bite. it'd inhabit wooded biomes (Pale Garden and swamps included. it can swim), mountainous biomes, plains biomes, caves, and Strongholds.

Stronghold: to improve it, the Stronghold would have things like forge, storage, armoury, dining hall, great hall, living quarters, and throne room, on top of what it already has; and it'd be a bit more spacious (the corridors would stay 3x3). simply put, it'd actually resemble an underground fortress. and as i said, the Troll and Wyrm would spawn there.

Ender Dragon: it'd actually try to kill you, it'd actively protect the higher-placed End Crystals, it'd land on the shorter pillars to spit 3 Dragon Fireballs, and would make use of its breath attack (the one it uses when perching on the Exit Portal). that's how i'd improve it.