Chance for buffed zombies to spawn.

Minecraft is a great game filled with different hostile mobs. However, it can get repetitive fighting the same creepers, zombies and skeletons. Late-game, it's really easy to fight mobs. 

A suggestion is to make it so that a zombie has a chance to spawn as a buffed zombie. This zombie is 2 blocks tall, slightly taller than normal zombies but can still fit through doors (maybe they're 1.99 blocks tall). They do 50% more damage than normal zombies and walk a bit faster. On normal mode, they disable shields for 3 seconds and on hard mode for 6 seconds.

The chance for a zombie to spawn as a buffed zombie depends on the difficulty. On easy mode, they don't exist. On normal mode, there's a 3% chance for a zombie to be a buff zombie and a 6% chance on hard mode. 

I think minor additions like this can help make the game a bit more challenging.