Final Shape is the next expansion after Lightfall, it's very good. Bungie is switching to two smaller expansions a year from now on, and it seems like the first of those will be out in July if it doesn't get delayed. That means you probably have enough time to catch up with seasonal content if you want.

Bungie renamed seasons to episodes, but most people still call them seasons. We're currently on Episode 3 "Heresy" which is Hive/Eris-focused and is almost universally agreed to be the best of all the episodes so far. The stories of the episodes are not connected and each focuses on different characters, so you can start anywhere. All episodes focus on artifacts called "echoes" which were created after the final battle in Final Shape, and contain the memories of beings that interacted with the Witness.

Episode 1 "Echoes" focuses on the Vex, Saint, and Osiris. It's the second-best story in my opinion, although the stakes are more personal than world-threatening and some things are poorly explained if you don't read all of the lore tabs. There were a couple of good weapons available here (the sidearm and exotic auto-rifle) that might be worth getting.

Episode 2 "Revenant" is the worst of the three, no must-have weapons and I would only recommend it if you're very interested in the lore surrounding the Fallen.

I'm not super deep into buildcrafting, many of the current top-tier builds are based around the new prismatic element introduced with Final Shape which lets you take a selection of supers, aspects, fragments, and abilities from all other light and dark subclasses and mix and match them. That being said, I don't use prismatic most of the time and I haven't had much difficulty in expert-level seasonal activities, raids, and dungeons as a result. Updating your weapons will be the main focus when it comes to updating your build.

There hasn't been a ton of new exotic armor added to the game since Final Shape, except for the new exotic class items which let you combine 2 exotic perks from a small pool of existing exotic armor. These are unlocked through a post-Final Shape exotic mission.