Currently there is a C++ API for creating custom geometry, but it is kind of a one-off solution.
We need a more comprehensive API for supporting user generated runtime geometry. Such an API should be useable by us internally as well for generating runtime geometry from our own mesh file format (currently the asset import mesh utilities has its own helpers for this).
The justification is that from a usability perspective users should be able to do anything that we can do with internal API's, so why not unify them.
Custom Geometry API's should be sufficiently low level to support any kind of geometry the engine supports, which means that we should take some inspiration from Qt3D for this API.