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.
Monopods are great for wedding reception shots, where you have to move fast in and around people and obstacles, and grab a candid shot and go on to the next one. A Fig Rig or one of the active stabilizers derived from drone photography is also useful.
GoPros and their knock-offs are so ubiquitous now, you should have a number of them with custom mounts ready to deploy all over the venue in hidden spots for b-roll, cover, and cut-away shots.
Adjustable LED light panel for the cameras.
A Bounce card with silver on one side, black on the other.
Agree about tiny digital audio recorders. Essential, and not expensive.
Connect two mic clamps together; attaches your wireless stick mic to the same mic stand as the MC's.
Gaff tape and narrow carpet runners to cover cables with.