so dragons are pretty great, but an often overlooked thing is that they have an variant innate spellcasting trait that can give them a lot of really fun spells from ANY spell list that i feel is severly overlooked, and so here are a few fun ideas i had with this trait in mind:

surfer dude gold dragon they are an ancient gold dragon whose prefered alternate human persona is an surfer who uses a lot of old surfer slang, and he has the spells tsumnami to create an mighty wave to ride on, control weather for perfect surfing conditions and glibness so that he may practice his ultra high charisma and so that the players never find out this surfer idiot is actiually secretly an demigod, possibly even mind blank and more stuff.

old grampa death an ancient black dragon, with finger of death, ressurection, soul cage, similacrum so at his disposal he has an potentially infinite supply of undead servants, the abillity to bring somebody back from the dead, the abillity to create an eerie replica of an humanoid, and the abillity to entrap a victims soul, making them perfect as an NPC or advisor to an more powerful necromancer, sly and deceptive sort of like an hag. For extra spook you make him an dracolich

elder evil an powerful evil spellcaster with some of the most powerful spells in the game, with antimagic field, clone, feeblemind and maze he is able to destroy the psyche of an spell caster, trap somebody with maze (this synnegizes well with feeblemind), clone alows him to create an back up of hmself in case he dies (completely for free since dragon spellcasting requires no material components), and cast anti magic field which works great since nether his breath weapon nor his weapon attacks are magical, so he is able to counteract dangerous spellcaster by using this feild arround him, assuming his gargantuan size is able to fit within the 10 ft radius (it will, for the most part, but the tips of his wings or his tail might stick out when he attacks or flies arround), yes this may be an red dragon, but he is powerful in the ways of magic

the great achitect an mighty golden dragon, he knows spells such as temple of the gods, fabricate, stone shape, mighty fortess, hallow, wall of stone, guards and wards, forbiddance, galder's tower, druid's grove and the like, he is an expert at magically creating buildings and magically protecting/ warding those buildings and sacred places, he wanders the land in the name of the gods to create strongholds that ward against evil free of charge to any orginization that might need it, making shure to protect it from anyone who would want to harm it

chef le extraordinare an ancient silver dragon, has stuff like create food and water, heroes feast, goodberry, as well as a few buff spell reflavoured as food like holy aura and remove curse, he or she just goes arround and cooks things for people, becuase in the end like other silver dragons they simply love the company and companionship of humanoids

inferno an ancient red dragon, with glibness, planar ally, infernal calling and planar binding, they are incredebly adept at convincing the servants of tiamat to ally themselves with him by summoning the abishai. This could also make for an really really interesting gold dragon who has ether succumbed to evil or who is using the evil to destroy the evil

lust an adult white dragon whose only innate spell is polymorph, a spell wich it uses to trasnform itself into an animal and fulfill its mor carnal urges with the local wildlife population. Can be used to justify an large half-dragon population nearby, and could also work for other types of dragons but white dragons are the most animalistic

dream lord an adult green dragon with dream so that it may message people in its dreams, as well as geas to force people to do its bidding, perhaps you can even throw in an infernal calling spell for fun, so that he may manipulate the abishai, maybe modify memory to manipulate people and truly turn them mad, scrying to keep an eye on his minions and prey, so on so fourth

dark librarian an silver or black dragon who studies dark magics, has divinations spells such as comprehend languages and contact other plane, as well as a few more comspic horro themed spells like maddening darkness, sickening radiance, crown of madness, eyebite, hunder of hadar and stuff like that

old grampa decay sibling of old grampa death, this one had stuff like insect plague, blight, eyebite or flesh to stone, possibly even planar ally or summon greater demon or infernal calling, also of course he has giant insect

truenamer studies the werid magic of true names and as a result has a lot of spells about word such as divine word, mass healing word, power word stun, tounges, perhaps an at will vicious mockery, speak with dead or plants, feeblemind, hex and glibness. Should be an mysterious fellow, maybe an gold dragon, but any type of dragon will do

great gardener an green dragon who is less evil than his kin and can cast stuff like awaken, speak with plants, spike growth, wall of thorns, transport via plants, entangle, entangle, grasping vine

got any more ideas? please tell me i would love to hear them, got any personal experience playing in an adventure with dragons or DM'ing an adventure with dragons, also tell me, please do!

also, it seems like when an dragon uses the change chape action option, it looses all properties of its dragonic physology (claws, bite natural armor, breath weapon) but retains all its magical aspects such as lair actions, legendary resistance and of course the change shape action, so by that pattern it is implied that dragons retain their spellcasting trait whilst transformed and so an metallic dragon could cast similarcum on themselves while in humanoid or beast form assuming they have that action option and similacrum as one of their innate spells (or they have an wizardly friend who can cast it for them), so add that to the list of cool shit an dragon can do with their magic, as well as that also meaning they can disguise themselves as an innocent rabbit and then cast delayed blast fireball on an evil king, then survive the blast whilst still disguised as an innocent rabbit.

you can also customize these further by giving them cool unique lairs that fit their individual personalities and adding spellcaster levels to them, such as necro wizard for that old grampa death or several levels in druid for that gardener or perhaps you want to make an copper dragon who has many many bard levels, this is just scrathching the surface of what you can do to make dragons more unique and interesting

also an silver, gold or green dragon, who has spells like find familliar, find greater steed, conjure fey, conjure celestial and stuff, peguasus, an spellcaster who is all about conjuring animals