When I first played Sunless Sea I didn't click with it, then the Zubmariner expansion/DLC came out and I was hooked for weeks. When I first played the arthropoddy Metroidvania-with-hints-of-Dark-Souls, Hollow Knight [official site], I didn't manage to click with it, but chatting with friends who adore it once it gets going and seeing that its first free DLC - Hidden Dreams - is out, I'm thinking I'll try it again and hope for a Zubmariner-type revelation!
Hidden Dreams is the first of three free content packs for the game so it sounds like there's a whole bunch to reinvigorate the game if you've been playing since release (i.e. the opposite to me) and are up for a new challenge/new things to see.
Here's the description:
"This is it gang! It’s time to face 2 of Hallownest’s greatest warriors, uncover a hidden stag station, and traverse Hallownest in a whole new way with Dream Gates! We’ve even included a few small surprises to keep veteran players on their toes."
So having not played much of the original the secrets might be a bit lost on me, but it sounds like Team Cherry have tried to pop some Easter Eggy elements in for players who mastered the original as well as the new bosses, a couple of new music tracks, a new Stag Station and so on.
If you're using Steam to play you'll update to the right version automatically, if you're elsewhere you're looking for the 184.108.40.206 update and the new bits and bobs will be integrated into your game.
One other noteworthy thing is that the Hidden Dreams update also brings with it an Italian language option thanks to fan translation efforts. There's also an attempt to fix some controller issues but that's in beta at the moment rather than being entirely stable and vouched-for.
You can read the full patch notes here although obviously I can't tell whether I need to include a spoiler warning for anything. I don't *think* I do as a couple of the notes a kinda cryptic so I assume Team Cherry are censoring as much as they need to themselves.
My fave note is: "Increased damage for Glowing Womb hatchlings"
P.S. I had no idea there was a little Hollow Knight modding scene! I'd only seen the cut content mod but there's also an uberdifficult mode called Glass Soul where you now die in a single hit and there's Lightbringer which converts your character's melee attacks to ranged ones.