I don't think a useful "shell from wireframe" group is too hard. Take the lines/edges in 3d, and find all the planes defined by some set of adjacent edges. In each plane, find all the closed loops, and those are your faces. Then test that the faces make a watertight, non-self-intersecting shell, choose their normal directions to be consistent (like by choosing one arbitrarily, and then flooding out from its edges), and then make sure the shell isn't inside-out (like by computing its volume, and flipping all the normals if it's negative). Curves get trickier, but I doubt the plane version of this would be more than a few hundred lines of code.

@whitequark, This is looks like common triangle mesh editing mode like it done e. g. in 3d studio max. We are defining the common 3d entities(segments, circles, arcs) and introduce something like "3d polygon" request which creates planar face from set of looped edges and perserves points to lay in the same plane using solver equations. This is like direct modeling in the context of single group of SolveSpace's history based approach. We can also introduce watertight and solid checks in the same way like it done for checking closed contours.

Hm, this is a very interesting idea. We can add a "direct modeling group", where you would use parametric entities, but unlike regular draw-in-3d groups, the group would also include a solid, defined in the way @jwesthues described above. I think this solves all UI concerns I had.

Would it be OK to have a checkbox on sketch-in-3d groups that causes a shell to be created from the entities in the group? There would be limitations on what you can create a shell from, but the UI would just be a checkbox. This shouldn't be hard for straight edges and convex objects.

The restriction to convex objects seems quite unfortunate. Does it buy us much?

I still hope to see this feature in SolveSpace. For most of my use cases, it would remove the need for lofts (e.g. lofting from one square to another is just a matter of drawing connecting lines and converting the whole thing to a solid). Even if it's restricted to planar faces only, it would still be extremely useful.