One of the biggest downsides to Disney's purchase of the Star Wars franchise was the the death of the Expanded Universe, the part of Star Wars canon that encompassed all of the books, video games, and stories that had added to the main Star Wars story over the years.
Admittedly, there were a lot of confusing elements in the EU from years of storytelling and re-telling, among conflicts with the prequel trilogy and other stories that would have made writing the sequel trilogy impossible. Those probably needed to go but it's a shame that some other parts of the EU had to go with it.
For me the biggest losses lie in Timothy Zahn's Thrawn trilogy and the KotOR series. Both are exceptional stories with rich characters that really added something to the universe that I feel even the movies have failed to do (looking at you The Force Awakens).
The good news is, there's hope. Thrawn is now canon again thanks to Rebels, even if he isn't nearly as well written and has yet to accomplish any of the things that made him the master tactician from his novels. At least his reintroduction has given Zahn the go ahead to write a new, entirely canon, Thrawn novel.
In that same vein of hope, although we may never see Revan become canon (unless he's Snoke) we at least have the fact that Kylo Ren's design and character HEAVILY borrow from the greatest lord of the Sith. The KOTOR style Hammerhead style cruiser also has been reintroduced in Rebels and Rogue One. So I can definitely see more of KOTOR being reintroduced. Hell, we might even see a return to the series at some point.
There are a whole lot of other things that I'd love to see return, including Mara Jade, but we'll take it one step at a time.
These people shared on reddit some other things they'd love to see return, what about you?