The Most Powerful element

Chlorine maybe? Nitrogen or oxygen would be good elements to control.

If that is really not what you are after, maybe a meta element, she can control other elemental's abilities?

 

Depends on what exactly you count as an "element." Once you let stuff like light and darkness count as elements, basically anything can be an element. (Usually I prefer to stick to the classical four, since they correspond to the four stages of matter: Solids, gasses, liquids and plasma.)

I dunno... How about Void/Nothingness? That is to say, her element is the antithesis of all other elements, letting her for example create voids where other elements can't exist.

Also, this reminds me of a story I was working on a while back, which had a magic system where the basics were made up of the classing elements. Each of them had a field of magic associated with it. (Air was movement, fire was destruction, water was transformation/restoration and earth was creation/reinforcement.) Different mages had different elemental affinities but a fully trained magician was supposed to have at least passing skill with all of them. After that the elements became more and more complex - the highest forms were necromancy and time/space magic, I think. The MC's family was particularly good at space magic, so their houses always had doors and rooms that didn't follow natural geometry.

Also, one of them figured out how to use antimatter magic. (Which wasn't too useful in everyday life, but really good for blowing stuff up.)

 

Maybe the ability to combine two elements: earth and fire = lava, for example. It's of a rare talent/ability for a wizard in my First Civilization's legacy world to have. It might work for your circumstances.

 

I think this page might help: Infinity+1 Element - Television Tropes & Idioms
As has already been mentioned Void is a classic super element, but if you don't like that, there's plenty of other options, Space, Time, Life, Death, Chaos, Law, Spirit, Mind, Heart, Sound or Song if you want to be more thematic. Or you could go with the opposite of Void, in the from of Creation, she can make stuff out of nothing. Or how about Alchemy, she can turn one element into another.

 

I also haw an elements based magical system and one of the more powerful elements is decay. Time could be powerful to, or as was suggested by TWErvin2 a combination of two or more elements.
Doodle Devil has some hilarious "elements" :tongue: