A really cool mob that would be extremely difficult to code for (I presume) would be something along the lines of a "shadow player". The easiest way I can describe this would be "Link vs. Shadow Link", but instead, it's the player (you) vs. a shadow variant of yourself. Ideally, the shadow variant would be able to copy/perform most of the moves a player can do. While I think this would be a cool idea, I don't actually expect it to get added at all, because I can only imagine the long string of complexities one would have to address.
Although, if you've heard of the Citizens plugin for Bukkit, I could imagine their code would make coding this new type of mob much easier (the plugin essentially spawns players that are modified to be NPCs).
And now for an idea/question that's not only for novelty purposes. So, when one thinks of a dungeon, one thinks of the stereotypical traps: sawblades, swinging blades, dart traps, etc. If you ever decide to implement a moving trap like a sawblade, or a ceiling that extends downward until it hits a solid block (AKA a crusher), are they going to be implemented as a block, as an entity, or as a block with a tile entity?
The Stun potion effect, as I read it, renders the player unable to move, and mobs unable to attack.
In the spirit of giving players a hard time, how about a potion effect that renders players unable to swing their arm at all (but they can still move), and akin to "Stun", renders mobs unable to move?
Or you could change it so that Stun renders both players and mobs are unable to move, and this new potion effect renders both players and mobs unable to attack.
Here's another idea. "Cursed" artifacts, talismans, equipment/weapons/armour, or some other items that inflict negative effects (whatever you want it to be) on you, whether it's because it's simply in your inventory, or that you equip it, or it gets used, or any other scenario. All in exchange for some "powerful boost" or "powerful ability" or whatnot. I'm sure the idea is pretty clear; most RPGs have something like this.
The effects can be anything ranging from simple damage and potion effects to aggressing every mob within a 4 chunk radius. How fun.
Stun already does that, or at least the one in the 1.7 version. I'm still working on stopping the player from moving their hand since that is done in rendering, and I'm also going to try to make it that you can't move the camera when you're stunned (have it hold in place the entire time you're stunned, for a true stunned feeling). However, you can't break blocks, attack or do anything while stunned.
I also already have a Harmony potion effect that makes almost everything unable to attack (mobs that use projectiles are unaffected by it I believe, as well as the two vanilla bosses) and doesn't allow them to move. The player can still move around but a few of the mobs have different movement methods that bypass it. (this isn't that big of a deal though)
The "Cursed" artifact thing was already planned for the stronger amulets I'm adding, but I guess that would be a good way to specify them. The amulets I'm doing are done in levels, lvl 1 has a weak effect, lvl 2 has a slightly stronger effect, lvl 3 has the full effect with an added debuff/buff or something. The "Cursed" ones could be already up to the tier of a normal lvl 3 amulet, but have a negative effect to counter that. Trading ease for risk.