I had imagined there was someone above the Five Elders in terms of ultimate authority going back to the conversation they had with Akainu at the end of Dressrosa. I was however surprised that the Elders are themselves Celestial Dragons. I had imagined that the Celestial Dragons were the ruling class that wield ultimate authority, but would not busy themselves with the minutia of governance. Thus, I had imagined the Elders, were tasked with governance by the Celestial Dragons, but were ultimately the highest ranking government officials likely coming from the lower king class. I had imagined that there would be a 'king' or leader of the Celestial Dragons who would wield ultimate authority over the entire world, but wasn't involved in day to day governance. I believe those old posts are in the "Final Villain" topic from 2015 - 2016.
The reason I bring this up is that my understanding of the revelations during Reverie is that this relationship is essentially correct, but the parties are different. The Five Elders are the highest ranking of the Celestial Dragons. Their names haven't yet been revealed, but I imagine they're each head of one of five of the families descendant from the original 20 kings - and perhaps the significance of those family names will be revealed once we learn about the Void Century.
I feel like Im is a secret to the world because his existence is directly related to the events of the void century, the erasure of the ancient civilization, and the ascendance of the 20 (actually 19) Kings. We don't know yet how many people within Marijoa are aware of Im's existence. Doflamngo indicated that he knows of a secret treasure. It's unclear yet whether that treasure is the Straw Hat, Im, or something else. It's possible Im is only known to select individuals, but is definitely completely unknown to those outside of Marijoa.
So here's what I imagine - the Five Elders are stated to be the 'world's highest authority.' Im is not at all concerned with or involved in the minutia of day to day governance and thus, the Five Elders are top dogs in that domain. However, I do not think it would be accurate to call Im a figurehead, which has the connotation of a monarch only nominally wielding power. Pulling out a game of Thrones quote, power resides where people believe it to reside. It is clear through their reverence that the Five Elders believe Im to have power. Even if no one else in the world bows to Im's authority. Im's introduction used visual cues to establish the power relationship between himself and the Five Elders. No text box saying "King of the World" was really necessary. Oda often shadows figures or delays formal introductions. Im didn't speak in that scene, so a floating exposition box serving as formal introduction might have been premature. A figurehead is ultimately powerless, but a secret figurehead really wouldn't even make much sense. The scene makes clear that the Five Elders who rule the World Government bow before Im. They regard Im as their ultimate authority.
What the Five Elders do is run the World Government. The World Government, Marines and Seven Warlords included, is an apparatus, an instrument used to maintain order. We can glean from the Elder's conversation that maintaining balance, order, and stability is of paramount importance. The question then needs to be asked, why they want to maintain balance. And I think the answer lies with Im. Specifically, I believe the established order is maintained to serve Im's goals or purposes. The World Government is to Im what Orochi and Wano are to Kaido. What Kaido wants is to create a global war. To prepare for that war, he needs Smile. To manufacture Smile, he needs Wano with its natural borders. A ruler like Oden would never allow a pirate like Kaido to exploit his country and a man like Orochi could never best Oden on his own. Kaido is completely unconcerned with ruling Wano. He doesn't serve as Shogun. Rather, he assigns that authority to Orochi. Orochi then serves Kaido because his authority rests on Kaido's power. Orochi is in one sense a figurehead in that his power is granted by a greater force. But in truth, Kaido, that greater force, is disinterested in the minutia of Orochi's tasks. It's almost like an owner and general manager relationship. Kaido is the 'owner' of Wano and Orochi 'manages' or rules as shogun.
I'd posit a similar relationship between Im and the Five Elders / Celestial Dragons. The World Government was created by 20 Kings who banded together to destroy the lost civilization. They erased 100 years of history and ascended to rule over the world as gods. I imagine most of them truly believe this to be the truth. After 800 years, I imagine the truth of Im is lost to most of them. Only the Five Elders, Im's vassals, and perhaps a few other high ranking officials (Kong? Bowler Hat Aegis Zero guy?) even know he exists. In a sense, calling Im the 'World King' might be a bit of a misnomer as it would imply Im cares about being king. Im is willing to sit, unbeknownst to almost everyone, on the empty throne. Which tells me, Im is not concerned with the tasks of governance or with recognition as the world's authority. Kaido cares not for the respect or adoration of the average Wano citizen. As Orochi maintains order in Wano such that Kaido can work toward his ultimate goal, the Five Elders and the World Government work to maintain the balance of power in the world while Im works toward his ultimate goal, which is at this point a mystery.
There are a few further indicators that this could be the case. First, we have to consider our protagonist and the potential implications of the True History. Whatever message the Poneglyphs contain, they must be able to inspire Luffy to action. Luffy is a character completely unconcerned with ancient history. He didn't even care to hear Nojiko tell the story of Arlong and Belle-Mere. He has already seen first hand atrocities committed by the Celestial Dragons and World Government. None of those things have been enough to set his sights on tearing down those institutions of power. He fought CP9 to rescue Robin. Once she was free and they escaped Enies Lobby, he didn't harbor any revolutionary sentiment. Luffy's goal is to become Pirate King. His ambition is motivated by his core desire to be the person in the world with the most freedom, the most autonomy. Dragon's introduction in Chapter 100 is accompanied by a quote from Gol D. Roger "These things cannot be stopped. Dreams. An inherited strength of will. The ebb and flow of the ages. As long as people hunger for freedom, these things will exist." We do not yet know the moment where these words were spoken, but I have a separate "Roger Incident" theory, which I can elaborate on at another time. I believe that Dragon's motivation as a Revolutionary and Rayleigh's statement that the Roger pirates were perhaps too hasty to challenge the world are likely related to this quote, as is the fact that the whole world knows his treasure by the name 'One Piece'.
I believe, whatever the true history reveals, it must be more than events long passed. Luffy won't care. It won't motivate action against the established order of the world as seems to be Roger's unfinished will left to his successor. I think the True History reveals not only the fate of the Lost Civilization, but also an ongoing, continuous threat to the world and the people Luffy cares about. Think of Father from FMA. The disappearance of the lost civilization is like the death of Xerxes at the hands of the Homunculus. Yet, Father's ambition remained incomplete. I don't know what exactly Im wants, but I imagine his ambition is incomplete. He needs the world to remain stable while he works towards that goal. He was likely instrumental to the 20 King's ascending to rule the world and now the Five Elders commune with Im as they work to maintain that balance. Their authority is granted by and dependent upon Im's power as Orochi's authority similarly relies on Kaido's power. There's a reason Dragon did not move to attack Marijoa while Sengoku, the Three Admirals, and Seven Warlords focused all their attention on Whitebeard at the Battle of Marineford. Shanks was facing off against Kaido. Aegis Zero and Kong are likely deterrents against aggression aimed at the Holy Land, but Doflamingo implied that the authority of the Celestial Dragons is dependent upon an ancient treasure (Im?) located in Marijoa. So, if Luffy's primary motivation is to seek freedom (chapter 500…something when he tells Rayeligh the reason he wants to find One Piece), and so too seemed to be of paramount concern to Roger, I believe Im and his ultimate goal / plan / desire likely represent the antithesis of freedom. The World Government, as it stands, is unjust, but not draconian. While we have seen the Celestial Dragons commit isolated acts of atrocity, by and large the citizens of the One Piece world do not live under an iron fist. Rather, the World Government is focused on balance. The reason stated for a great cleansing is not to oppress the citizens of the world but to re-establish order as the world has remained restless for the two years following the destabilizing events at Marineford.
So, in summary, I believe Im wants the opposite of what Luffy wants. Luffy wants true freedom. Im likely wants the opposite of that for the world. The World Government, as it presently exists, doesn't really represent that - they're bad, sure, but not an overbearing evil presence of oppression for most citizens of the world. One Piece and the title of Pirate King are Luffy's goals, yes, but not his motivating drive. The World Government wouldn't go to the lengths they have to cover up past events that have no bearing on the present. The true history must reveal Im's existence and ultimate desire, which threaten the lives and freedom of Luffy and the people he cares about. One Piece is the key to stopping that ambition. When the treasure is discovered, the world will be turned upside down.
I have some additional theories to go along with this, but this is generally the power dynamic I would imagine between Im and the Five Elders. The Five Elders are mortal and the present heads of the World Government. They have lived normal lifespans and ultimately would choose successors when they step down or die among the heads of the 19 Celestial Dragon households. The present Five Elders have been around for a while, but haven't ruled for all 800 years. Im has been around the entire time - from the Void Century to the present.