I have a number of important NPCs; mostly sheriffs and other officials the party might meet as they stroll into town.  But the party has one key NPC that matters more than all the others so far and this is the caravan leader they are travelling with.

For safety, people travel in groups on the roads between town and the party has latched onto a merchant caravan.  They are hired as extra security and they get the use of a horse to ride, a single cart for other party stuff, tents and free food.  They also receive 2gp per person, per town or city (nothing for villages or hamlets) as they travel.  The caravan has five "old men" able to defend the caravan, and six "young men" able to defend the caravan.  One of the Old Men is the Caravan Leader, Illihambur.  Except ... Illihambur is a Copper Dragon that is passing himself off as a human from another nation the caravan calls home.  The people in the caravan know about this, but the party does not know.  The party thinks he is a high-ish level Bard.

I am trying to plan for the big reveal when they learn he was a Copper Dragon masquerading as a bard and not a bard that once used Polymorph to turn temporarily into a Copper Dragon.

The caravan is about twenty wagons long.  The party travels up front with one member of the Caravan that "knows the way."  The caravan is often threatened by Griffons.  All the fighters in the party are equipped with a crossbow, except for one ranger who uses a longbow.  So when the Griffons show up, everyone dismounts and starts shooting the griffons with arrows.  In an earlier encounter, on round three, the Griffons had just landed next to a wagon and were threatening to capture two horses when a Copper dragon landed near the Griffons and scared them away.  They missed their attempt to grapple a horse and flew away empty handed.  Four griffons were killed, and two got away.

Based on the way things have played out, I am pretty sure the party "figured out" that Illihambur's class is Bard and he is at least 7th level (the party is currently 3rd level, and they were 2nd level when this attack happened) so he can cast Polymorph.  In the evenings he often has an informal celebration with music and drink around the campfire.  He has friends where they travel, and they are often Bards.

So when the Caravan goes south and the party goes north, I want to "reveal" that he was a copper dragon, not that he could become a copper dragon.  He doesn't want regular folks to know about this.  This is why he travels incognito.  His name, Illihambur, is very well known and folks look forward to his caravan coming through their town.  They sell tobacco, a fine tobacco like a Cuban Cigar, but in this world it is called Tocco.  So he has an interest in keeping the caravan front active.

Another Griffon attack is probably going to happen in the next session.  This time there will be ten griffons because it will happen close to the town as they depart.  The town guard will also shoot their bows at the griffons.  But Illihambur isn't going to reveal to the townsfolk that he can polymorph into a copper dragon.  He will confide in the party if they ask why he didn't "handle it" that he has reasons for not revealing his powers to the average commoner, so they would do him a favor not to share knowledge of his powers.

What sort of ideas do y'all have for handling the big reveal?

There is a lot in here, and I suspect you probably have an idea already in your head for how you want to do this.  My advice would be to dig into Fizban’s Treasury of Dragons for potential inspiration on what would prompt the reveal.  Is this dragon protecting something very special, like his horde, or an egg? Is there a rival that threatens him, or threatens to ruin him by exposing him?  Is there some great obligation he has to fulfill?  Find the strongest motivator you can imagine for what he values, vs. why he tries to stay hidden, then write the necessary plot points to put those two things in immediate conflict and force his hand.

The Copper Dragon (Illihambur) is aligned with a group of good dragons that are keeping watch over this part of the world (Material Plane).  He was sent by this group to be an ambassador to the nation where the caravan comes from to let the authorities know a group of dragons are watching out over this region.  So most of the people in that nation do not know Illihambur is a dragon either.

After establishing relations with this nation, a new nation among men arose, which goes by many names, but flatters itself as The Great Nation on the Lake.  The "lake" is an inland sea like the Black Sea.  The Great Nation on the Lake (GNotL) is the first Nation of Men to be established on the North Shore of the lake/sea.  This placed them in direct conflict with a nomadic confederacy of Orcs.  The arrival of these men upset the balance on the north shore.

Illihambur decided he would also want to make contact with these men and try to advise them to stay out of trouble (the kind of trouble that comes from visiting other planes).  He attached himself to a merchant caravan and travels throughout the GNotL.  The members of the Caravan learned about Illihambur's true nature but they understand they have nothing to gain by revealing this.

I don't plan to reveal anything more to the party at this time other than he is a dragon that walks in the disguise of a human rather than being a bard that can polymorph into a dragon.  I think the reveal will leave the party wondering among themselves why (this dragon is bothering with them) and this will be the source of many interesting conversations.  But more important will be the seed planted that what they see may not be what is, and what they think they know may not be true at all.

Earlier in this adventure already a silver dragon flew to an important city the party was passing through with the Caravan.  The Silver Dragon alighted on a local peak and spoke to the group that came out to meet him, including the Baron, Illihambur, an important diplomat named Bene Timber and the party, and many onlookers.  The Silver Dragon spoke a prophecy that a great plague (that are Gnolls, but only named a Great Plague by the Dragon) is descending on the land.  The humans of GNotL are well advised to make peace with the Orcs so they are not faced with two great foes at once.  Then he flies off after he is flattered and thanked by Bene Timber.  Bene Timber knows all the big players, including Illihambur, and he leaves soon after to formulate a plan to make peace with the Orcs, but the party doesn't know this.  The party only knows he rode off soon after.  The party will be invited to become an instrument in this plan.  Right now, the party only knows about the prophecy that a great plague is coming and the Silver Dragon advises they make peace with the Orcs.  Every time it has been discussed since, with low level commoners, the reaction has been something like "I'll be dead and gone before we make peace with beasts."

The next stage of this story arc comes when the party and caravan arrive in the city of Pegasus, which should happen in our next session.  The party has been steadily moving north and west toward the frontiers where the Orcs lay beyond.  In Pegasus, the leaders will have an excuse to summon the party to ask if they would help make contact with the Orcs to discuss relations.  The party has a Bard (my niece) and I hope that the example of other Bards acting like diplomats will inspire her to push to accept the job.  The party has a Ranger (my Sister-in-Law) who is well suited to guide the party in the region.  One of her favored enemies is Orcs and her favored terrain is woodlands which occur in abundance on the frontiers between the men and the Orcs.  The party contains two elves (my wife, a paladin, and my son, a land druid & experienced 5e player) that are historically in deep enmity with the Orcs.  My brother is a human fighter, and the spokesman for the party (as an old AD&D player) and my nephew is a dwarven war cleric.

That is the broad brush of the story as it sits right now.