I think a better mesh tool is key to ArchiCAD. We would like some better tools for creating site analysis for digging out sites for foundations and other various site work tools.
Add-ons for Foundations would be great! Like you have for Duct Works, etc.
You could have tools for setting up typical Pile Cap/Footing, gradebeams, continuous footings, column (with base plate and anchor bolts would be awesome), pile and so on.