Probably some kind of energy field bending. It'd be pretty neat if you could alter the temperature of surfaces and move electrons around to make electrical currents and such. The implications of energy bending could be very god-modey though, theoretically you could rob enemies of all their body's natural energy and prevent them from fighting back. If I had to be more fair, maybe I'd condone bending of plants.

I could picture you going bulbasaur on your enemies, controlling vines, making the leaves fly off nearby trees and become like airbourne blades to swarm your opponents and cut them to ribbons.

Anything else I really can't think of, because it either falls into another category or is too obscure to be useful. Ice bending should be its own school, it seems like it has a very different mindset than basic waterbending. Waterbending is very much about upper body and arm movements. I could imagine icebending being a lot like earthbending, in a cave of ice or something.

But yeah, two cents.

i would want icecream, pizza, and mcdonald fry bending that way i could make icecream, pizza, and mcdonald french frys any time by makeing them out of everything. i would probably end up fat but it woud be so much fun. and so tasty. oh and i would also like to have chocolate bending. How cool would it be to coat yourself in choclate than slowly eat yourself lol. that would be so awesome.

that would be awesome coating myself in chocolate is on my "things to do before i die list"

I figure if we move to the modern age Denim bending would be an incredible power. I mean, EVERYBODY wears jeans, you'd never be away from your bending substance. Also you could create some incredibly durable shields. Or simply rip a jeans-clad enemy's pants off and render him embarrased and useless.
But I have to reply to something Zazbaz said in that 'theoretically energy bending would be Godmodding.' It's sort of funny that he'd ignore the originals when 'TECHNICALLY' blood bending IS God Modding incarnate. 'Theoretically' I'm sure anyone could think of a godmodding way to use any bending form. You know a sort of [Insta-kill ZOMG ^_^] move.
[spoilers]Also if you haven't seen Sozin's comet you've already called the 5th and invented the 6th. Plot Twist Bending.[/spoilers]

Much as I hate to discourage creativity like this, I am saddened to see that several of you have not read these rules which are sticky-posted right in the Bending Arts board. Even the idea of an additional element suggestion thread would be okay, if it was serious, but since this doesn't appear to be a serious conversation, I don't see why this thread should be open in the first place. I'll leave it unlocked for serious discussion but otherwise this thread will be closed and archived.

Serious discussion, eh? Well, let's see....

To start with, it's a little tricky trying to invent a fifth element, since the traditional four cover most things very well - fire takes care of all energy forms, earth takes care of matter, water takes care of liquid, and air takes care of gas. Still, where there's a will, there's a way.

The most obvious candidate for a fifth element would be something like time, a la Prince of Persia. Naturally, this would be an incredibly potent ability on it's own: however, a little tweaking would make it an equal partner to the other four elements.

For instance, you could make it exponentially harder to timebend based on how far backwards or forwards in time you go, as well as how much you try to affect. Also, since you're bending time, rather than breaking it, you couldn't use timebending to "skip" a period of time - you'd have to do something like slow time down around you so that a year seems like a day - potentially a very dangerous move, especially if someone finds you in this state and decides to finish you off.

Also, rather than affect an entire object, you could affect a small part of an object, but at a greater rate. So for instance, you could cause a small portion of a sword to age very rapidly, making it rust away. Alternatively, you could throw up a "time shield" - an area in front of you where any missiles (arrows, fire, water etc) that pass through it are slowed right down, giving you time to dodge clear. A similar technique used on yourself could slow your descent to earth - although that would raise the issue of whether bending time helps you cheat momentum.

And that could be another candidate for an element there - momentum. Doesn't let you levitate anything, but it you set something moving... well, let's just say you're gonna be one deadly archer. ;D