In the campaign I'm running the modern-day characters PCs just had to go into a fantasy-like metaplane, and my plan is to have them deal with a dragon in the Shadowrun style--more like an evil genius/mastermind than a smashy kaiju. Always bugs me when games have dragons that are just sort of killable through pretty standard means.
In my setting, I've split dragons into two groups: Drakes, and true Dragons. Drakes are your big beasty; dangerous, need to slay them, the basic. They're intelligent, but not hyper so. They don't have a culture. Just big, dominant monsters.
Dragons, on the other hand, are akin to gods. But, that doesn't mean omnipotent gods, as my setting is animistic and has tons of lesser spirits that are seen as gods. Dragons are just one kind of such a god.
It varies wildly.
In my last campaign dragons played..... zero role. The party only met two.
*Fairly early on they defeated the White dragon that had moved into the Phandalian area. It was mostly a fight with a dumb brute. They were supposed to learn that something was going on with Giants, thus the young dragon relocating to the area. They missed the connection between rumors of Giant activity & the influx of monsters into more civilized lands. Well, until very very late in the campaign.
*At one point the PCs interacted with Limryth(?) - the polymorphed Blue in SKT - in her Storm Giant guise in a non-combat social situation. But they failed to pick up on the clues as to what she was/or maybe they didn't care. Either way they ended up being manipulated by her.
The campaign before that? They came to the attention of the red dragon in ToA & eventually fought it in its lair.
In the coming campaign? Dragons will either be an important factor (Odyssey of the Dragon Lords) or little - none (a pirate based campaign)
Varies. Sometimes they are just slightly smart brutal beasts. Sometimes they are scheming and grand opponents. Really all depends on the campaign. Generally the older they are, the more important they become to the overall campaign.
I use two kinds of dragons in the Ashen Lands as well. The "True" Dragons and "Lesser" dragons.
The True Dragons are very rare, and represent your Chromatic/Metallic dragonkind. Narratively they're aloof and hidden, rarely seen.
Meanwhile Lesser Dragons are something you might come across while adventuring in the low to mid level range. Full grown, not babies, they don't get Lair Actions, or legendary actions, and are really just very large and somewhat cunning beasts. To the point where they can be raised and trained as flying mounts. Particularly -dangerous- flying mounts... But the smaller varieties only get up to about horse-sized and largely feed on fish and things smaller than your average wolf.
In one that I'm currently running, the party is busy protecting three rapidly aging (for dragons) brass wyrmlings that they "liberated" from a fey area where a contract that their mother (now dead) had was to "keep my babies safe", and the archfey kept they safe and babies. So instead of foes, they have been a long-term goal of the party to teach them from the nigh-feral wyrmlings as they regain the decades (centuries, really) that they had been frozen in age.
In another game I run the dragons are conspicuously extinct, with the skull of one of them suspended below the roof of the Imperial Library. Dwarves are also extinct, genocided for their access to "The Bones of the Earth" (literal) and replaced with a magically created race - the drow (who are completely different in culture). However, it's determined that the land is near magic-dead (spells above 6th just won't work anymore), drained from centuries of high-magic civilization. There is another place though that has it's magic, and the players may yet find dragons there.
Last campaign I completed was 13th Age using a moderately homebrewed version of their Dragon Empire. Dragons showed up as foes many times, and the primordial Chromatics were fairly heavily involved in the big picture dynamics of the world, and some with the party itself who had connections to them. They also had to protect a young and headstrong dragon from assassination attempts against the families of the dragons supporting the Dragon Empire.
1) The party fought 2 or 3 (forget which) adult dragon skeletons in the treasury. The party was about 10th level at the time and had a paladin and a zealot barbarian with a maul and GWM. The skeletons didn't last long. Not really an end boss, but definitely unusual for the party.
2) (spoiler)Dracula turns into a vampire shadow dragon as a second form, because this is Castlevania without the TM, dangit! He didn't do much talking. The breath weapon was his main source of damage. It was an epic fight, but he rolled poorly. End Boss.
Current campaign (Aztec Aaracokra):
1) Young adult black dragon swims up to the 13th-level party's small ship at night, intending to sneak over the top, roar, and extort loot. The rogue on watch spots him, starts chatting in draconic while the dragon is still setting up, and ends up humoring him, flattering him, and handing him 1,000gp as the price of passage through his territory. Half of the party wanted to kill him, but didn't want to jump in and mess up the negotiations.
(Had the party fought him, he'd have gone "Oh, oops" when multiple magic weapons lit up and hit him, and then run away).
I was very surprised but it was fun.
Not an end boss, but more an opportunity for the players to feel the difference between level 3 & level 13 tangibly.
2) (spoiler) Ancient modified green dragon guarding an artifact. Very plot relevant, enough HP to last through 5 rounds of combat, and a bit arrogant. Not an end boss but certainly a major fight, especially since he started the fight invisible and nobody had True Sight or See Invisibility. The Paladin ended up getting Hasted, Misty Stepping onto the dragon's back, and then Smiting him repeatedly.
He's still alive, and the party may end up trying to hire him as an ally for a major battle. Not sure.
3) The other dragon in the region is a mid-CR white. At this point (party about to hit 15) he is not going to be a deadly challenge for them. I'm not sure if they will end up bribing him, hiring him, or killing him.
So, to sum it up:
1 End Boss
1 Regional Boss
2 Solo Monsters that may not be fights
1 Set of Undead
Action economy issues are still a thing, especially since the stock Draconic legendary actions are only useful if the dragon wants to be in melee. Smart dragons don't want to be right next to 3 melee PCs...they want to be doing hit and run, casting spells, or using their breath weapon.
Dragons are definitely not a creature you want to over-use.
The first box set LMoP had the green dragon in it that the party fought to a stalemate initially and them killed it later in a follow-on quest around level 8. That game also had another dragon in it that the PCs fought and then ran from in the mountains. Current campaign is Icespire Peak with another dragon as the main threat to the region. It makes for a good game plot to explain the orcs and such, but has not come up in the game yet. I plan to make it a better encounter than just kill it at level 6-7.
I guess most of the campaigns I run have a dragon eventually, but mostly are levels above 10. They are a bit like a major demon or an angel or such in that they are rare and hard to find but when they are around everyone knows about it.
I normally don't use dragons all that often, but my previous campaign centered around a dragon that was pretending to be the returned king of a large region that had been prophesied. So the dragon had badgered/threatened some lesser dragons into helping him, but for the most part he was just the power behind the distant throne. He was also using his own mother (she was not a willing participant) to magically create half-dragons of various forms.
Funny thing is for various reasons they never actually fought the dragon (although they did fight a few others). They managed to find and free mommy dearest and she chased off her imposter son.
As a general rule the older the dragon, the more tactical and less likely they are to ever get into a direct confrontation. Young or even some adult dragons are more likely to engage directly, but that tends to weed out the stupid or unwise ones. The ancient ones? They're far more likely to use others as cannon fodder and to have schemes that unfold over decades and centuries.
In battle, all but the youngest are never going to get into melee with PCs if they can at all avoid it. They'll use hit-and-run tactics using either breath weapon or bombarding from on high. They'll also do everything in their power to set up unfair fights; if absolutely necessary they'll flee to plot revenge and fight another day. It also depends on the type of dragon we're talking about of course. White dragons tend to be brutes, black dragons are schemers and so on.
In my current campaign the group is in the teens and have yet to face a single dragon. Not sure they ever will. On the other hand I do have a dragon war (led by mommy dearest and her now subservient son from the previous campaign) that's kind of rattling around. Maybe next campaign, with part of the campaign enlisting the help of the good dragons of the world.
In my games, dragons are largely unintelligent creatures that work to destroy the status quo of whatever region they inhabit and impoverish it by stealing all the treasure for themselves. Once civilization in the area has been reduced to a level they find “primitive” enough for their own personal satisfaction, they actively patrol said region to enforce the new status quo.
This is a result of their original creation by the gods as elemental conquerors and usurpers.
My dragons aren’t there for you to make conversation with. They are there to be fought and killed.
In my world, they're the head race of the Draconic Empire (which also includes Dragonborn, Kobolds, and some Lizardfolk). Whether I specifically involve them as enemies, or allies, or neutral powerful NPCs differs between campaigns, but they mainly are mentioned in their relationship to the Giants (whom they had a centuries-long war with), or their current war with the Aboleths and Krakens (which they actually cooperate with the Giants on now, with a few giants actually being allowed to ride ancient dragons into combat).
My current campaign is all about dragons. They took over the continent several hundred years ago, and now they occasionally just fly in and take over towns. The PCs are dedicated dragon hunters and each adventure is a hunt to take down a different dragon.