Ever since Disney bought Lucasfilm, they have been establishing a new canon for Star Wars. So far the explored areas have been between Revenge of the Sith and A New Hope, between A New Hope and Empire Strikes Back, and finally between Return of the Jedi and The Force Awakens. One can see how it might be tricky to explore that timeframe. Usually between movies there is a timegap of at least three years. However this timegap is about a year and with a very specific mission statement. At the end of Empire, Han Solo has been captured and the others promised to rescue him as they knew exactly where he was being taken to. But there are enough significant changes in Return that can be explored.
Luke Skywalker arguably has the biggest change in between the movies. At the end of the former, he was barely skilled in the ways of the Force. By the beginning of the latter, he is highly knowledgeable and a deleted scene that was supposed to open the movie has him finishing the construction of his lightsaber. Now did he get that way? Did he return to see Yoda? The talk between the two is vague enough where he could have returned to resume his training. In the new canon, Luke is established as reading journals left by Obi-Wan Kenobi and these have progressed his training.
Lando Calrissian is the second character to overgo a major change. He was last seen as the pseudo replacement for Han Solo but not actually a member of the Rebel Alliance. We learn later that he had achieved the rank of General and was a well respected member in it. How did he excel so fast? What did he do? You can assume Leia had something to do with it but the rank of General is very far up. His exploits about what he did to become so well regarded and known is something to explore.
Leia and Chewbacca are two who probably don’t really need to get fleshed out. It seemed pretty obvious that their main goal was to get Han Solo back. Of course Leia is someone who will never let a man define her so it wouldn’t be a stretch to see her continuing to work with the Rebel Alliance. She could be the glue between Luke and Lando’s story. It would be interesting to see how the group actually infiltrated Jabba’s palace at the beginning of Return as Jabba doesn’t seem like he would just let anyone match in.
Finally there is Darth Vader and the Emperor. The two’s relationship has been explored throughout the new canon. At first it was simply Darth Vader hoping that the Emperor would be his new best friend while the Emperor knowing this and playing on this but seeing Vader only as a tool. In the ongoing Star Wars and Darth Vader comics which are set between A New Hope and Empire Strikes Back, Vader learned just how expendable he is to the Emperor and how he is working towards making Luke his apprentice to take over. How is their relationship during this time gap? The Emperor always seems to know what Vader is feeling and up to. Vader is seemingly resigned to his fate by the time he and the Emperor meet in Return. We also know that there is a new Death Star being constructed and the Emperor is planning a trap.
The time frame between Empire and Return is a small window of time. However a lot can happen in a year. We do see that for ourselves. There is still much to be mined from the time period. It is only a matter of time before Lucasfilm will cover this themselves. It will certainly be worth the wait.