I'd like to suggest a feature that most won't give two tosses about but which will be well regarded by a few.
Basically, to have the option to define a set of signatures that will show up depending on what theme the viewer is using. This way it will let those user conscious of colour schemes have the peace of mind that they will always fit in nicely.
I would imagine it to work in such a way that the default is to define a global signature and for those that really want to there is the option to define separate ones per theme or to have the option to use the global one.
In order to conserve some storage space it might even be worth being able to define styles for the different themes while keeping content intact (think CSS). The way this would work is that the user defines the content in the global signature entry and can use standard tags such as h1, h2, etc as well as a set of user styles, possibly just with tags such as [user1], [user2], etc. Each of these could then be define on a global or per theme basis (with the theme definition overriding the global one).
Another useful feature would be, for example, to define changeable content. That is, have a tag that gets replaced with content in the theme definition. This way pictures with different colour themes could be inserted in the correct location relative to text content.