We’ve been having a lot of fun with the dragons from Fizban’s Treasury of Dragons in the Coliseum. It’s got some great items, but by far my favorites are the gem dragons.
The problem with regular dragons in D&D 5e is that they are relatively mundane. They don’t have magic weapons. Unless you follow variant rules, they don’t have magic. They don’t have magic resistance. In my opinion, dragons from the Monster Manual are radically underpowered. I’ve had people make the argument that dragons are intended to be hit and run monsters, and shouldn’t be expected to stand their ground in a fight. I can see their point, but I think a dragon should be dangerous.
Gem dragons have magic, exotic damage types, and most importantly, a bonus action teleport. This is key. It makes them dangerous as hit and run fighters. They can mix it up with PCs at melee range while their breath weapon is exhausted and get away without taking an opportunity attack. This is critical especially in case one of your players has the Sentinel feat. The last thing you want is for your dragon to be grounded for an entire turn while your whole crew of players hammer on it.
Fizban’s Treasury of Dragons Has Greatwyrms!
Another solid magical upgrade for your dragons is the Greatwyrm. The metallic and chromatic versions dwarf ancient dragons in power. Check Arcane Lance, newly added to my list of favorite Legendary Actions: