I don't know if Sony has something like it in the pipeline, but Canon used to offer a 200-400mm f/4L IS USM Extender 1.4x, and Nikon has recently released two of them: the NIKKOR Z 400mm f/2.8 TC VR S and the NIKKOR Z 600mm f/4 TC VR S. They're frighteningly expensive (!), but they're ideal for wildlife photography.