I don’t have as many ideas for how to “improve” ROTJ, but one simple idea I have in mind is to restructure the first half hour or so. In a nutshell, I would do a bit of a sequence rearrangement like this:

Sequence 1: Vader landing on the second death star
Sequence 2: Luke on Dagobah
Sequence 3: Jabba the Hutt

This would give the film more of a beginning, middle, and end, as the basics and rules of the plot are really explained on Dagobah. Starting the film off with Jabba the Hutt was not the best decision, because that entire episode had almost no set up for the rest of the film. The Jabba episode is better as a part of the second act, not the first.

The only issue that concerns me is how to explain Luke’s talk with Han in the X-Wing after the Jabba sequence ends. This is because the scene is a prelude to the Dagobah sequence, so getting it to transition to the rebel base could be tricky. Other than that, I think it’s an improvement.