Who? Snoke probably. But an angle from afar that leaves their height ambiguous could potentially have it be Luke too.
What and why? I just enjoy the concept of ending with Vader’s castle. It’s both commemorating the end of Anakin Skywalker’s story, and giving us a small amount of foreshadowing that he could still have influence on the future of Star Wars.
Sounds like a fun scene to edit as a stand alone project, but it doesn’t seem like it belongs in Rotj. Especially without a clear idea of who, when, or why.