There are very few expansion packs that felt like they lived up to the standard of the original game, just thinking back to the short period in time when they were being sold as boxed releases a few of those come to mind. Mysteries of the Sith for Dark Forces 2 was one of them, the gameplay is more of the same with a couple of cool additions and the levels are fine, but they skimped out on the FMV cutscenes and used in-engine ones instead so we never got the 90's edgy cheese of
. Or take Alien Crossfire for SMAC, they did add some nice new buildings and techs, but the new factions weren't as great as the old ones and felt very superfluous, game didn't need more of them, and not the progenitor aliens either. I guess people weren't used to nickel and diming people back then, so when it came to making expansion packs, be it by the original studio or not, there were often not much to do, game was already complete and all ideas they had were in them.
90's shooters had generally good expansions since it's a hard formula to fuck up if you already have a good core, just add a couple of new weapons and enemies with a level pack constituting a new campaign. The Plasma Pak for Blood is a good example of that and it was a good sendoff to the game, letting you blast through a final hardcore episode that integrated a couple of cut things from the game. Return to Na Pali for Unreal had Legend do the same for that game, taking cut maps and knitting together more Unreal, even if the story had really ended. Marathon Infinity is remarkable since it themed the story in the game around the release of an official mapping tool and was even better written than the base game.
The shooter expansion I was most impressed with was Wages of Sin of SiN, mostly because it was made by another studio but managed to keep up the very high standards Ritual had set with the original game, another campaign about as long, non-linear levels that change depending on your decisions, the amount of interactivity and a plethora of new enemies, textures, and new items and weapons. At times it felt like playing the Duke Nukem Forever we never got with the casino level. The only real continuation we got of the first game too since the sequel fell for the episodic meme that never ever worked out.
Real-time strategy game expansions were similar, the standard being some new units, perhaps a new faction, a new campaign and some skirmish maps, but I can't think of any that really stood out as being particularly great, not that many of them were bad. Like Winter of the Wolf for Battle Realms, it doesn't innovate much. Homeworld: Cataclysm is noteworthy for being a stand-alone expansion pack that diverges slightly from the original, but not entirely in good ways.
Even after reading through this thread I can't really think of any expansion pack that is a
must have or that radically improves on the base game. In terms of expansion packs that truly changes the game the only thing that comes to mind is Dinosaur Digs for Zoo Tycoon, because it's an entirely different vibe to tend to a zoo full of dinosaurs a la Jurassic Park than it is to keep a normal zoo. The best you can hope for is an expansion pack it seems is one that feels like a sequel done cheap with the same assets and engine of the base game.