Hmm....
Earthbending: Control metal. Just as Roax said: magnatism.
Waterbending: I dunno'. Call water vapor from the air instead of using a nearby body of water? They could maybe super-condense the vater vapor...THAT would be awesome.
Airbending: Fly without a glider, for one. *cough* NO, I WASN'T JUST WATCHING YU-YU HAKUSHO!!

The Waterbenders can already take water vapor from air (look at Katara in the episode "Jet")
You're right for the Earthbenders, if they could control metal it would be awesome.
Airbenders flying without glider ? I think it's already done too but you gave me an idea : what if they could stop the time ? it is a crazy idea but it's still an idea...

Airbenders stopping time? Don't see how that's possible. I know they could trap someone in like an air vortex, or slice someone with a slash of air, which are moves that aang hasn't done. Has for waterbending, It takes a lot of energy to take the water from the air, so they would probably be weakened in a fight if they were forced to keep using it. How about Firebenders Controling Heat, has in body heat, friction, or other various sources. I think it would be cool if they could make explosions by using thermal energy to some source,.

How many times has it been shown that a difficult move become less effective if a bender keeps using it?

do you think if a fire, air, or water bender were skilled enough they could concentrate the element so much that it makes it almost solid, allowind them to use it as a weapon (and wih water, i mean without freezing it). Damn, a bender would have to be like, better thanthe avatar him/herself at their respective bending.

also, I don't get why firebenders can control lightning! is it because fire and lightning are both hot, energetic phenomenon? sorry for being sorta off topic.

Or firebenders speeding up kenetic energy? *coughGambitcough* And can slip through time and space, even gain hypnotic abilities? Yeah, I'll stop.
I'd be interesting if there were ETHER benders. Like...controlling people's souls and stuff. It'd be necromancers! Vast armies of the UNDEAD!
"Those emo freaks?!"
*begins singing* "Crawling in my rooooooobes, these wooounds require elixer!"
Yeah, you can shoot me now.

well when you think about it both fire and lightning are just reactions they are not pure substances so yeah, or the writers just thought it would be coold who knows

fully trained Avatar creating thunderstorms by using all the elements

That would be cool if they could make lightning storms! or wait can the avatar do that... or am i imagining things? well any ways i think it is odd that they can control lightinig. I guess ( even though this is dumb cause im not into sciencey stuff)its cuase they both are heated elements like sparks make fire. And if you were to strip fire of the elements it would be made of some of the same things as lightning...WOW that sounded smart even if im not right! oh yeah ( a while back) someone mentioned Tera off of TT, that is very possible... Flying by rock.. i'd be afraid to fly *coughs.. im Not Afraid Of Heights!* on a rock unless my feet where strapped to the rock...But yes it is very possible. Also about the metal bending thing, it would be kinda hard... Most ore is in the mantle and thats far away from the top of the earth for a person..Wow i guess i am smart! Unless you were in a mine your out of luck..

for the Chi question boomy called it jin.
and for the bendings couldn't air benders make giant tornado's and thrash everything.

Bumi refered to it as Jing, which in chinese refers to ones essence. In taost philosophy jing becomes chi which becomes shen.

this about possible bending arts..not JING...., well gotta go on vacation now!!

I would think the Avatar could only do a few things, like magma, storms, becasue storms are Air+Water+Lightning, and either needs a lot of benders, or the Avatar.

Yes and magma (dispite what wikieplidia says) would requre Earth and Fire. Thought I think air is the major factor in storms. (aang in the episiode "Imprisonment" that he wish he could have summoned a big hurricane to scare away the guards.)

Well, since Aang i the avatar, he could make a tornado with rain and everything. Clouds, rain, lightening, strong winds.... the whole shebang.

I'v been thinking. With the avatar's powers he could make crops grow FASTER!!! Water, earth and FIRE!!!.

How's fire mix into that?

Ooh, something the Avatar could do- short order chef!! He could cut with water/ice, boil with water/fire, stir and cool with air/water, cook with fire.... Oh thatd be great! McAangs!