@ManipUni:Visio is a great tool but it actually does too many things: I think it would improve a lot if it could be only about diagrams (there are better programs for floor diagrams, etc.). This could actually make it a lot cheaper.

I think a different tool could be very interesting: how about an Office converter that allows to import DWG, SVG, EPS into DrawingML? This would allow to use more specific tools to create the drawings/diagrams and them use these directly into Office programs.