In no particular order, here are a few gems:

• Age of Empires II: The Conquerors (2000) - AoE2 was already one of the better RTS games ever made, and this expansion built on it swimmingly. You had a bunch of quality of life changes, new visual elements such as snowy and tropical terrain, improved AI, an expanded tech tree, new maps, new campaigns, five new factions, and the new Uniquer Technology mechanic. There's a reason why the game survived for over a decade without support.
• Fallout: New Vegas - Old World Blues (2011) - Basically an entire Fallout game in miniature, complete with interesting quests, a decent story and roster of characters, new items/enemies/perks, and The Toaster.
• Mario Kart 8's content packs (2014 & 2015) - $12 for 16 high quality new tracks with original graphics and music, 6 new characters, 12 kart parts, and 200cc? This was a heck of a bargain. As a bonus, the Animal Crossing pack came out weeks early, something that hardly ever happens in this industry.
• Shovel Knight: Specter of Torment (2017) - A new campaign and delightful set of mechanics alongside remixed music to match the new tone.
• Sonic & Knuckles (1994) - This one is a bit arguable, but I'm including it here because buying this let you add new content to Sonic 2 and Sonic 3.

1. Everquest: The Ruins of Kunark
2. Everquest: The scars of Velious
3. Everquest: Lost Dungeons of Norrath
4. Everquest: Omens of War
5. Baldur's Gate: Tales of the Sword coast
6. WoW: Burning Crusades
7. WoW: Wrath of the Lich King
8. Burnout Paradise: Big Surf Island
9. DriveClub: Bikes

Then there is No Man's Sky. It's still giving me new content for the original disk copy, and all in VR now. (I did upgrade to digital in a sale years ago, still have the original day 1 disk)

Man, you must have a whole in your soul that's the shape of an everquest title. :). Have you tried Final Fantasy 14 online? It's the new standard for MMO's or at least it wad the last time I played in 2021, still. You might get some of that magic back cause the community ther me is AWESOME. Alas, I know your not gaming till the winter. 

My old favorite series Wing Commander had a good deal of expansions and all of them nice. Maybe WC2 Special Operations were the best of them, but WC:Privateer Righteous Fire and the WC:Prophecy independent DLC (included in Gold edition) Secret Ops were better then the already great games themselves. I can't quite say that for WC1 Secret Missions, but WC1 is my absolute favorite game ever.

Baldur's Gate and Icewind Dale had better and worse. Throne of Bhaal is the best part of old BGs and I really liked IWD1's Heart of Winter expansion and especially the DLC expansion of expansion, Trials of the Luremaster.

Another good old one is Lucas shooter Outlaws. The Handful of Missions isn't a huge expansion, but it's a nice addition to just story and multiplayer.

And I won't argue on AoE2: Conquerors. First AoE expansion Rise of Rome wasn't bad either.

Everquest is still going. If I go back to MMORPG gaming, it will be Everquest ;)

The 31st expansion is scheduled to come out in December. There is also project 1999 if you want to play the classic experience again.

AoE 3 expansions weren't bad either, I remember enjoying them but that game seems to be hated on for some odd reason, I am one of the few who thinks it's better than AoE 2. 

Yeah I thought the War Chiefs was pretty decent but I probably enjoyed the Asian Dynasties the most. But the campaigns for both expansions were great!

AoE3 is considered the worst by AoE2 purists lol. Some of it stems from Ensemble’s founder that said how they tried to change a ton of things about the series, but tried reverting too late, so there’s aspects that are half baked. Like the home city and XP system feels very tacked on and really fleshed out. Then card system is another matter entirely to purists lol but AoE3 is the most fun in the mainline series and the one I played the most.