For Leia though, making a couple of tweaks to my initial thoughts; you could very easily have near the film’s start had a scene with her arguing with the Alliance command about the necessity/feasibility of rescuing Han, (or just her storming out afterwards even) and perhaps have her demoted for insisting on going to rescue him. Rescue ensues, et cetera et cetera, it now makes more sense for her to be on the Endor shuttle crew.
Add another scene if you like of her taking charge during the battle from one of the Imperial communications relay or something after the loss of some of the rebel leaders due to the Death Star II, or their communications are knocked out, if you want a further pay off for the earlier scene.
These two additions would not take more than a couple of minutes of screen time and BAM you have an arc for Leia without even altering the plot (because sorry, I did not like the recycled use of a Death Star much, cool as the space battle was).
Sorry, bad ideas again. First of all, the rescue as it is was already decided/initiated in ESB. The last scene is basically dedicated to that (they send Lando ahead etc.). Secondly, her making such as stupid argument would actually ruin her character. She always seemed to give priorities to the rebel cause at expense of any personal stuff. In the end there is no necessity/feasibility in rescuing Han from the rebellion perspective. It is a waste of resources.
Now all this speculation is pointless, other than to point out that it would have been very much possible to give the other major characters of the OT arcs in ROTJ, even within the existing plot structure of the film.
Both Han and Liea still have major character arcs in ROTJ. They just aren’t as dumb as you would like them to be. 😃