Disc and face primitives
Well, maybe you also want to introduce some "flat" shapes, like discs and 4-sided faces?
I don't see a need for this ... and there is a reason that most 3D programs don't offer this. A disc is really just a cylinder that is flattened or has had the sides and one cap cut off. So, you already have this with the cylinder tool. A square is just a cube with the five other sides missing or it's flattened. So, once again, you already have this in the 3D shapes that are there.
The way some 3D programs overcome this is by how you create your shape. For example, to create a cylinder, you drag out the size of the cap and then drag up to get the length of the cylinder. If you only wanted a flat cap (i.e. a circle or disc) then you would only draw out the cap and drop the tool (i.e. you don't drag up/down to create any length). To create hexagons, pentagons, etc, you simply have an input box to control the number of sides a cylinder has.
So, instead of cluttering up Curvy's UI with tons of shapes, these flat shapes can already be made from what is already there. Curvy just needs a better way to do it.
More import & export formats
Some more standard formats should be introduced in Curvy 3D 4.0. I imagine at least .3ds, .wrl, .lwo, and .stl. The latter is a very common 3d printing format.
More export formats would be welcome, I think. But the ones you list are not ones I would recommend. .3ds is old, outdated, and extremely limited. For example, a .3ds file cannot hold more than 65,536 vertices per mesh ... not polygons, but VERTICES. Meshes must also be all triangles ... no quads. Accurate vertex normals cannot be stored in .3ds. And there are quite a few other file format limitations, too. I see no reason to support .3ds. And as someone who works in 3D, it's literally been years (more than a decade at least) since I've had to even think about anything .3ds. I cannot see this being a file format that would be of any use to Curvy.
Why .wrl? Do you really want to export for VRML? Isn't VRML (mostly) dead? If you want, for example, to display your 3D creations on a website, then there are much more robust 3D methods than VRML. I would recommend that Curvy export to glTF instead.
.lwo is a Lightwave specific file format. Why would we need such a specific file format? OBJ or FBX would do us much better here as you could get out everything you need from Curvy and into Lightwave, if that's what you're looking for. Curvy already supports .obj, so only FBX would need to be added. However, from what I've heard there are some issues with .fbx and Autodesk. There are even better solutions available, though. Once again, glTF is a better solution. It is like a better version of .dae (Collada) that can get your model to various 3D programs, to the web, and into game engines. It's a bit newer, so not everyone is supporting it yet, but it's the future of 3D file formats. The Unreal engine is in the process of adding it for getting 3D game content into their engine, Godot (a 2D/3D game engine) already supports it, Facebook allows you to upload glTF files (with size restrictions) to show off your 3D models to people on Facebook, MODO imports/exports glTF and other modelers are in the process of adding it.
If I were to recommend Curvy add any new file formats, it would be: .glTF and possibly .dae as these will get your model, textures, and materials into just about any 3D program or game engine.