The Modify Solids tool will be increasing used as more and more needs to be modeled in 3d first for BIM purposes. It would be very very useful to add a few more options in addition to distance. See OpenBuildings Designer's Extend Linear Form tool.

I would also include an associated +/- offset parameter as well. This would allow the Solid to be extended to another face or plane or edge with a set offset for a joint (-ve) or key (+ve)