1) Signature character and image limits.
I find many of my users to have too many images or too much text in their signatures. Enable us to limit users to 3 images and 100 characters.
2) Image sizes in signatures.
Limit the size of images that are allowed in signatures, so that someone can only have 500x500 or under.
3) Signature size.
Cut a signature off so it can only be say 50px high, regardless of how much text/images are there. (this can be done with templates)
4) Hide Signature for mobile/responsive.
Do not show the signature for users on a mobile device or displaying the forum under x width.