Anything that represents a UI element, for example:
charting extension; cfchart, cfchartdata, cfchartseries
display management tags; cfdiv, cflayout, cflayoutarea, cfmap, cfmapitem, cfmediaplayer, cfmenu, cfmenuitem, cfmessagebox, cfpod, cfprogressbar, cftooltip, cfwindow
display management tags
From memory, we choose not to support display management tags at all. However, it might be an area where we cherry pick one or two to showcase how extensions are built and at the same time add a bit of additional ACF compatibility.
A lot of work could go into a decent charting package that vastly exceeds the existing CFCHART. Perhaps we could leverage some of the HighCharts work that Gert did year's ago. Or possibly there is a community group that might want to tackle this.
Again it might be another great "example extension" that offers a simple non-java based option for extending the core engine.