Giant zombie as a miniboss.

My idea is that in low probabilities the giants can spawn in the plains (to avoid being blocked by the blocks or trees of other biomes) or as a miniboss in the zombie siege.

They would attack not with their arms, but by stepping on us. 

Being so tall it would be difficult for him to notice what is on the ground so the player could get very close to him.

It could drop an enchantment or something that makes the player can damage the mobs when falling on them. Like Mario!

Maybe when he walks you could make the screen vibrate and make a big noise.

Instead of the typical zombie sounds it would have more low pitched, long lasting sounds that would be heard from many blocks away.

Because of its size it could easily walk over terrain several blocks high without having to jump.