From a practical perspective, I could easily see how having Luke find a crystal floating in space could be cobbled together in a simple way, rather than needing to create a new sequence of Luke going on a ground mission.
You could reuse various shots of Luke in his X-Wing that you can intercut with. You could take one of the shots of R2 in the X-Wing, and add a claw element that can grab onto the crystal. Maybe someone could do this with CG elements, but I also could see someone using blue screen to film the claw grabbing the crystal (crystal on a string, maybe?), then adding it into the shot.
The biggest thing would be creating the “Death Star debris” elements. This is something else you could possibly achieve by filming “debris” elements behind blue screen, using similar techniques that they used to create the asteroid sequence in Empire. If it were me, I think I might take pieces of styrofoam and repaint them to look like chunks of metal, then film them in front of a blue screen with my camera on a track.
I think if you did this, you would also need to establish that there is a giant crystal in the Death Star in the first place. Maybe you could take the reactor shot from ROTJ, and add it into ANH, but with a giant green crystal in the middle. You could also perhaps add a giant crystal into the Death Star II as well. Or, maybe the Death Star plans animation could have a green diamond shape in the center of the Death Star to indicate that it is the power source in the middle of the Death Star that the proton torpedo overloads.
Although I don’t necessarily think this is a essential idea, recently I thought it would be interesting to explore reediting Return of the Jedi so it is in a chiasmus structure with A New Hope. Basically, Return of the Jedi would have story beats that parallel A New Hope, but in reverse order. So, for example, the first scene of Return of the Jedi would parallel the last scene of A New Hope. So maybe you could parallel the Throne Room ceremony in ANH by placing the Emperor’s arrival at the beginning of ROTJ. Then you could create a new final shot of ROTJ, a blockade runner flying overhead, that could reflect the opening shot of ANH. Basically, the imagery and narrative beats would come full circle.
Bingo’s idea of Luke returning to the site of the Battle of Yavin would fit rather well with this idea. If it presumably came after the first scene (whether it be Vader or the Emperor arriving at the Death Star), Luke flying through the Death Star debris would reflect the second-to-last sequence in ANH, the Battle of Yavin.
I would be interested in exploring this idea of making Return of the Jedi reflect the narrative beats and imagery of A New Hope further if anyone else is interested. If anyone else is curious about the chiasmus idea, Robert Lockard did some articles about it a long time ago. Here is his article about Empire Strikes Back and how it has symmetrical story beats from beginning to end. He also did an article on the Back to the Future trilogy, and how the whole trilogy has this narrative reflection as well. Interestingly, the middle film, in both the case of Back to the Future Part 2 and ESB, ends up reflecting itself once it reaches the midpoint (both of that single film, and the trilogy as the whole). In Back to the Future’s case, Part III ended up reflecting Part I. In other words, the end of the story reflects the beginning. So it would be interesting to approach a potential ROTJ edit with this in mind, and how it might be possible to have the end (ROTJ) reflect the beginning (ANH).