Assuming the camera(s), glass, lav mic, tripod, lighting, batteries, memory cards, extra memory cards and more batteries are already taken care of:
1. A giant pouch that I can clip to my belt. I use the case that came with my 70-200 lens. It carries a smaller lens, batteries, ND filters, lens caps, all my memory cards, and a spare shotgun mic.
2. The aforementioned ND filter (an absolute essential only if your camera doesn’t have an ND filter built in).
3. Spare XLR cable and external recorder, like a Zoom H4N or H1. I had an XLR cable go bad in the middle of a wedding once and had to rely on the audio captured by a DSLR camera. Not. Good. Not having an extra cable and external recorder is a rookie mistake I will NEVER make again.
4. A slider. It’s a pain in the neck to lug around and set up, but the little extra motion it adds to your shots really makes a difference.
5. Shoulder mount kit. During dances the tripod will severely limit the shots you can get, and any stabilization is better than going handheld. If you can afford a decent gimbal then go for it, but you can find a basic shoulder mount for $100.
Honorable mention: Gaffers tape. No videographer’s kit is complete without a roll.