Visual language, a new tool
As for future development of plain language, I think we would benefit from what Dave Gray of VizThink describes as “cross-pollination between visual thinking disciplines like information design, information architecture, data visualization, geographers, graphic facilitation/graphic recording, comics/graphic novels, architecture, etc..” Plain language needs to incorporate “visual language” but first we need to learn what it is.
Also, at Brains on Purpose a blog post by Stephanie West Allen offers many other points of access to this field: http://westallen.typepad.com/brains_on_purpose/2008/04/images-paint-an.html