If you have a 24-70 f/2.8 lens, you have one of the best zoom lenses available, bar-none, provided it is the correct focal length.
The biggest problem with this type of video is usually the perspective (placement of the camera). Shooting it from the middle of the room, with a wide angle lens makes everything tiny, flat and generally really poor looking.
Most professional shoots, when they want to get the whole stage, will position the camera very high in the back of the room and in the center, or possibly slightly to one side. Is there a balcony, or a ledge you can put it on? That will look much nicer. The further back you can get (and use a correspondingly longer lens), the better because the longer lens compresses the image and makes those distant dancers seem closer together and is a more natural perspective for viewing things on stage.
But if you don't have these options, then, in the end, you won't find a much better lens than the 24-70 for what it sounds like you're describing, unless I've misunderstood your needs. Seriously, a faster lens like the f/1.8 options would let in more light, but I don't think it would change the video quality much, if at all.
If you had a problem with the quality last year, what was it?
Video is low resolution, so unless you're saying that it was too dark, I don't think any change is warranted.
Was it blurry last year? Was it too dark? Or was everyone on the image just a tiny dot because it was set to record the whole stage from within the audience with a wide angle lens? heh