Regional difficulty (the difficulty of mobs in a chunk get harder the longer you inhabit it) was a really neat idea, but it's too subtle to make a difference. My idea focuses on increasing the difficulty of Minecraft over time, without just changing mob's stats.

With the help of /u/SendineisTheParadox, I've come up with a very unique set of mobs that add a whole new mechanic to the game.

The Watchling

A new hostile mob that has no actual way to attack the Player.

It roams the overworld at night, searching for Players. If it spots you, it'll focus a spotlight from it's pupil on you. This makes any mob in the area, even those out of view range, able to see you.

It will continue to track you until you kill it or break line of sight.

The Watchling won't spawn the first night, it only has a chance to spawn once the regional difficulty gets high enough.

When day breaks there are no mobs for the Watchling the alarm, so it will actively flee the player until the next night (or if it despawns first)

The Nether Watchling

A variation of the Watchling that's bigger and spawns in the Nether.

If it catches you with it's spotlight, it will aggravate all nearby Zombie pigmen.

It spawns naturally, regardless of regional difficulty (because the Nether's supposed to be a challenge), but also uncommonly.

It doesn't hang around a single area, it hunts from place to place and moves on if it doesn't find anyone. They make a loud, very distinct noise, so the best strategy is to avoid it is hiding if you hear it, until it leaves.

The Ender Watchling

The third and final Watchling that has wings and spawns in the End.

If its spotlight hits you, all nearby Enderman will attack you.

They spawn rarely in the End Islands, and have a similar behavior to the Nether Watchling, except they usually stick to their own island.

A swarm of angry enderman can be very difficult, but keep in mind that the watchling spawns in the End Islands, which can only be reached after killing the dragon. It's a very End game area, so there should be End game mobs.

Conclusion:

The one thing that the idea is missing, is what they drop.

These are pretty difficult mobs, so they should probably drop something valuable.

Perhaps they each drop something different that can be combined into one thing that spawns one giant eye boss? But then what does that drop?

Let me know what you think – and what future Proof of Concept mobs you want to see.

As always, I'll gladly give out the model/texture to anyone who wants to use them.

Other Proof of Concepts:

The Magma Cyclops
The Ent