This idea is aimed at the harder areas of course.
But I thought 'what about a mob that hides?'
A mob that either turns itself into surrounding blocks and when a player comes near it, it pops out it's shape shifting camouflage and attacks the player.
Say for example a mob that will destroy a few blocks of a tree's trunk and then 'shape shifts' into the destroyed blocks and replaces those blocks, then the player passes by and poof there it is. Apply this to literally any block in the world. This would be an interesting feature for new players to be catched off guard like this.
Alternatively a mob that digs through blocks, meaning one that goes underground. To cut back on the lag thsi might cause, it wouldn't actually destroy blocks while digging, but only when going into the ground and popping out of it.
How about it?