We're getting the Kylo Ren miniseries right now, which expands his downfall behind the stuff we learned in the films. It also sheds some light on Luke and Ben's training together, and how both know Lor Son Tekka, Max Von Sydow's brief character from the opening of TFA.
ROS pretty much has given some more openings for spin-off material which could be interesting.
-Luke and Lando's search for the Jedi hunter. It's an interesting pairing especially since the two didn't really interact that much in the OT. Also, how much did Luke suspect that Palpatine was back? Did Ben's fall cause Luke to abandon his search?
-Poe's days running spice with Bliss, and possible implied romance.
-What Palpatine was up to for three decades apart from what ROS reveals and how he shaped the first order behind the scenes (As I've noted, the origins of the Order from the EU seem to differ from what it eventually becomes).
-Perhaps most interesting, Palpatine's son.
-Exegol's relationship to the other Sith planets we've seen (Moriband, Malachor etc).
-Maybe, at some point, a post-ROS saga showing Rey rebuilding the order perhaps with help from Finn? Granted, the now Legends post-ROTJ novels were the "official" sequels for a while, and then pretty much got removed because Disney wanted to do it's own thing. So there's a risk there in case one day they decide to do another sequel trilogy.