With the prevalence of 3d motion capture devices and AR just around the corner for more mainstream use, it's only a matter of time before you will be signing documents with such equipment.
Previously, equipment would note the pressure and timing in which you signed your name uniquely with 2 dimensions
but why not give you the option to add depth to it as well?
Further, you could choose the boundary box so anything outside won't get saved as part of your signature.
What would you change if you had a 3rd dimension to add to your signature?
It can even get flattened on a paper document but stored in 3d elsewhere, making it that much harder for someone to forge.