Gem Dragons, or "what happens when you need more variants of a popular monster" aren't something that hugely jumped out as me as adding much to D&D, but I make a small exception for Amethyst Dragons. With their dislike of Far Realm monsters and a tendency to prefer to live on the Elemental Planes, they can serve as a strange ally and a good indicator to the players that things have gotten weird.
These Amethyst Dragon Wyrmlings seem to have been limited release models, so when I spotted them on sale as separates in a store, I grabbed them straight away.
Ssurrans are a variant Lizardfolk that were in Dark Sun, and Wizards of the Coast have revisited as part of Spelljammer. Rather than produce entirely new stat blocks, they've suggested using Lizardfolk stat blocks plus a couple of new ones in their Spelljammer book.
The Neogi are awful interdimensional slaving monsters that have no redeeming features and are therefore a good villain to run into. I first came across them in "Minsc and Boo's Journal of Villainy" as awful merchants in some evil city. They give off strong "when the Forgotten Realms got too big" energy to me, but really found their home as Spelljammer villains.
In both these cases, the weird look of the miniature and the organic shapes of them that suit Wizkids mediocre painting quality meant I was willing to pick them both up to use as odd monsters at some point.