The first monster I remember ever being taken by in D&D was the Gelatinous Cube. I was reading through the monster pamphlet that came with the original Red Box, and found myself reading about something that was essentially a carnivorous jelly cube. It struck me as rather absurd at the time, and to be honest, still does, though in a good way.
4e's Gelatinous Cubes share many of the traits of their earlier brethren - near invisibility, a paralysing touch and the ability to engulf their prey - but felt a little rubbish in comparison to them. To address this I have souped my Cubes up...and here is one souped up from the basic Monster Manual version. Enjoy
So, what have I done? Well, I've given them back their immunity to lightning, and have upped the damage to make it a proper elite brute. The Squeal power is designed to give the cube a chance to control mobbing adventurer's and feels kinda' right for a shivering, wobbly death jelly.
By making its Engulf attack more vicious when it's bloodied, the cube really feels like an elite, and becomes a truly deadly foe. Without teamwork, good rolls and maybe some acid resistant trousers, a characters absorbed by the cube could easily find themselves looking down the wrong end of a death penalty.