I have two things to say:
One regarding that mini-video posted by Rock earlier: Was that a Boomer? If yes, holy are they OP.
And about mob stacking:
What about some fair mobstacks for a change?
: Simply 5-10 Zombies stacked. For a twist, make the lower ones faster and the higher ones stronger.
: Start with a very slow mob. What about a spider with slowness? It can climb, but not do anything else, really. Also give it negative Jump Boost. Then add skeletons as you wish. Basically that'd be a very slow moving piece of pain, with a little bit of Dakka (not enough though).
A Little Bit Painful
: A creeper ridden by arrows. I'm unsure if it would work like I think, but if it does, you can have arrows shoot in random directions when the creeper explodes. And they fall straight down when he dies, so they block your way for a short time, if you just walk you get hit...
IT'S A TRAP!
: Boat ridden by any invisible ranged mob. Perferrably a witch, you go near, potion just appears in front of you and a few seconds later it's IN YOUR FACE. This would probably only trick you once, afterwards you will be much more careful when approaching boats/minecarts
Actually... Let's just add random potions
: Zombie ridden by invisible witch. The witch will randomly add some potion effects to the battle and can even sometimes heal the zombie when it throws a damage pot!
The Great Wall of NOPE
: This one is made for Mazes with special walls. Basically you have any mob, stacked several times, and let it spawn all over the place (wide-area spawner). Give the bottom mob massive slowness, the higher up the faster they get (say, from slow 5 to speed 5 in a 11-Stack). In a wide area these would form a dangerous moving pillar/wall of doom. For some unfairness, make them electric fields (aka charged invisible creepers)
: Skeleton on top of a Pig with speed 20. Just because why not?
€: Well, this is #50000. At the moment. There are 3 off-topic posts above me sadly. Still, I got it
And obligatory custom meme for this situation:
Yes, I used the same meme template twice. Dealwithit