Hi Matt,
did you work at Xara, in Gaddesden Place? I've been there, to meet Computer Concepts many years before it became Xara... but that's another story!
(Ever used TopModel 3D software on Archimedes? I was leader of the company which created it)
Regarding your question, this is another interesting package I loved in the RISC OS version (now available for Windows): http://www.davidpilling.net/ovationpro.html
And Cerilica Vantage had some tools and behaviours (line tool, arbitrary-shaped gradient fills, interactive documents) that can be quite inspiring.
A graphic engine like Truism 2 could be used as the foundation for the full range of software you can offer (new version of Designer, Publisher etc.)
Cheers
Paolo