Hierarchy of Visio diagrams
A longterm feature which the current UI couldn't support, but I'm just planting the seed...:
Multi-level Visio diagram, such that clicking on a sub-process would jump to the appropriate detailed diagram for that diagram.
Not sure how this could be handled by the UI, since 1 overview diagram would easily require 5+ sub-diagrams.
Two biggest use cases being for flowcharts and floorplans (drilling down an office's department displays the floorplan for that department).
David Parker commented