Probably an unpopular opinion, but I think the current 3D implementation is a pretty good fit for C3.
It's a bit heavy handed, but at the same time rather elegant how it makes 3D as easy as 2D, by introducing well thought out limitations.
No other engine would have even thought of the idea to implement 3D objects with only 1 axes of rotation, but it makes working in 3D feel just as fast, and easy, as working in 2D in Construct.
Even advanced developers still run into issues with 3D rotation sometimes, so taking that complexity out feels like a great fit for C3.
Another area that is solved elegantly is texturing, another area that even advanced developers struggle with.
Yes the 3D object selection is limited, but it also completely eliminates the need for UV mapping in C3, and making the default texture a 6 sided dice was also really smart. It's like a tutorial without any words.
With that said, there are some things I think could be improved to make everything a bit more flexible and more useful.
After using the 3D features for a while now, I do have to agree that a locked top down camera in the editor feels very limiting and awkward. If we could have a better 3D camera view in the editor that would make level design much easier.
There should be a couple more 3D shapes, sphere is an obvious one, maybe the texturing for it could be done via triplanar mapping.
There should also be a billboard 3D object, a 2D plane that's always facing the camera. With an option to only rotate on the 1 axes that C3 already supports, but also one where the plane is always facing the camera.
The "Corner (in)" 3D shape is wrong IMO, the edge of the (in) part should be switched (since it should be a counterpart to corner (out)), so that the crease actually goes IN. Right now it makes a triangle on top and slopes down from there. (Might actually be a bug that slipped through)
This is how it is currently
This is how it should be
Corner (in) and (out) shape should be rotated so that their highest point is in the top left corner. (so there doesn't need to be any extra conversion if you rotate it with an origin in the top left corner (to be consistent with other 0,0 origins))
And I would also argue that the Wedge shape should be rotated 180° by default, since the default 0° direction is to the right in C3.
There should be a triangle shape (if you look from top down).
(The triangles Catheti should face up and left, so that the 90° angle is in the top left corner. (again so there doesn't need to be any extra conversion if you rotate it with an origin in the top left corner.))
Something I kept making feature requests for is an expression to get the x,y,z coordinates of mesh points.
And if we could switch the triangulation of individual grid cells in a mesh that would make modelling things with the mesh less problematic. (See the "Corner (in)" shape as an example of the problem.)