While not a perfect expansion by any means, it's a DLC that introduces new stuff that can be built on, rather than torn down in future expansions.
World Of Warcraft's latest expansion, Dragonflight, introduced a lovely array of waterfall cliffs along pretty shores and beautiful mountain temples. Perhaps the two biggest things, though? A new race that actually lets you play as a dragon-person, as well as the ability to soar through the sky on the back of a customisable dragon. While not a perfect expansion by any means, it's a DLC that introduces new stuff that can be built on, rather than torn down in future expansions. Not to mention that it's a little more casual friendly too.