If you consider returning to the game, I would still wait a bit - the recent patch introduced the biggest addition so far: a turn-based mode for those who prefer TB over Real-Time with Pause. This seemed to introduce a host of issue no matter which combat style you use. Turn-Based is still in beta and we are all waiting to the next patch which will introduce some much needed polish to turn-based combat and squish all the nasty bugs.
As far as the base game is concerned there isn't anything new story wise. There was a lot of work done to polish the gameplay - balance and difficulty are better, some new helpful UI, like you can see what companions are recommended for each quest so you won't miss out on some core interactions. Dispositions and reputations work better. There is "Deck of Many Ships" the final of the many free DLCs, which add unique merchant ship which sells some unique items. Each class received a new subclass, but be wary - those don't have dedicated reactivity. If you come back making another complain that the Steel Garotte paladin didnt' get reactivity during the trial in Woedica's Sanctum I will find you, drag you out in the middle of the night, bring you to a forgotten temple and choke you myself.
For those who beat the game and want to replay it and spice things up are Margran Fires, which you can access by clicking on the Magran Head icon at the bottom of the main menu - those are modifiers to vary things up. Also multiple Mega-bosses were added, which will test your battle skills, sadly no story attached to them.
The biggest additions to the game are three DLCs - Beast of Winter, Seeker, Slayer Survivor and The Forgotten Sanctum. All of them are in-game expansions (a very major sidequests so to speak) and IMO all of them are worth playing. Each of them focuses around one of the sidekicks to flesh them out:
Beast of Winter - introduces new sidekick, Vatnir, and has a lot of reactivity with Yidwyn. It's a very nice, balanced DLC - a story drived adventure with some exploration of important lore, dive into culture of a tribe from White that Wends some memorable battles. My personal favourite of the pack.
SSS - a combat focused DLC, though more enjoyable then I expected. It is essencially an arena, but don't take it as criticism - it has it's own storyline and it is probably the most robust arena I have seen in an RPG. Just be ready to do a lot of fighting and at this point those probably will be the hardest encounters you fought. Sidekicks who gets a lot of reactivity here is Konstanten.
Forgotten Sanctum - the oddest of those. It is a dungeon, though more in Deadfire style than PoE1 - think Deadlight Fortress. It's is a really fun area, with bizzare ideas, grim sense of humor, some Wael goodness and some really really interesting lore teases.