One of the options a couple of my users have mentioned to me is the ability to not only plug in a static signature file, but also to have the ability to select a file from the hard drive as a signature.

For instance, they run signify, which uses a basic signature framework and mates it up with a rotating tagline. The next available sig is a pipe named .signature. Giving them the ability to select a file (e.g. $HOME/.signature) would make them happy.