The consensus in the modding community is that horseback combat is a pain in the ass to make, because of the NPC specific behaviour, navmesh, road meshes and agression status of the ridden creature. There is a mod that implement agression for horseriders in Skyrim, and while it's technically "working", it will never be as smooth and simple as in Witcher III. Maybe the best modders of the loverslab would have the skills to actually make it work like a charm, somehow, but the stress it will bring to Papyrus will probably make it incompatible with everything.
Not sure it can ever be implemented properly in Gamebryo/Papyrus, tbh. Since it's difficult even for modders, the chances of Bethesda doing it are pretty small, unless they start using a completely new engine. Although, a modder once made a functional horse riding/fighting mod for New Vegas, the thing is, nobody knows how he made it (and he cannot release it, since it uses animation assets from another game or something like that. If I remember correctly, Bloodironzero made the initial mod, and Dragbody enhanced it to include shooting mechanics)