Something I really want is the ability to write code to produce exact curves exportable via STEP, so I can get them into Fusion 360, which is what most CNC (e.g. the Shopbot PRS Alpha) prefers to consume. Imprecise polyhedra approximations are fine if you're 3D printing, but subtractive methods like CNC routers can do orders of magnitude smaller features. Bumping up the precision is just kicking the can down the road, also, most CAM software will just roll over and die if you give it shapes with massive polycounts, while it'll trivially handle the BRep version of the underlying shape (which might be a simple set of splines for example).

So far OpenCASCADE is the most promising library I've found. And it's been around for a long while: I don't remember the last time I had to tell bindgen about the "DECOSF1" macro!

And of course AutoCAD, but this is a hobby and I don't feel like spending thousands :-)