Can't comment on Unity but Unreal4 definitely allows this, although you may have patch the engine source or implement your own base classes etc. I would imagine

Can't comment on Unity but Unreal4 definitely allows this, although you may have patch the engine source or implement your own base classes etc. I would imagine Unity is not that much different. See Astroneer for an example of this, the planets surfaces (which are deformable by the way) are procedurally generated, even if they are deployed from a kind of template specifying the parameters. UE4 has a ProceduralMeshComponent for this sort of thing, not to mention procedural foliage component etc.