let's talk the walk and walk around...I don't...whatever, just let's get started!
For the very first time I am confident in my answers for this 15+ year old riddle, so I hope you will greatly enjoy this take. What is Dragon's secret and why is he being held back? I think the answer lies with his tattoo.
Its actually quite an easy tattoo to understand if we take the most flashiest part out of it, the lines. We actually have seen these lines on many occasions and they symbolize "containment".
We see this for example with Ryuma and how a message was contained within identical lines to Dragon's tattoo. Basically, 4 lines in form of a square are used to contain a message. We see this in other anime like the Gotei in Bleach.
Hence why we can safely discard the lines and focus on what is contained within them, Red cubes.
The angle when the Road Poneglyph was introduced looks 2-dimensional, this is similar to Dragon's cubes, who are also 2-dimensional. That is a weird way of illustrating Poneglyphs to say the least and I estimate it is a hint to Dragon's tattoo.
"Can it really be that Dragon is related to Road Poneglyphs? "
Almost certainly, his entire path of taking down the world government is a kin to Luffy's and as a follower of the "will of D" he is connected to them already. He is also aware of the state of the world and defined the mentality of "the night" that the One Piece world is metaphorically suffering under, a way of thinking that the "D's" resist against.
Of all the characters who have no obvious relations to the Roger Pirates, Dragon seems to be the most woke guy alive. Rebels think so and have joined him in a battle for freedom. The revolutionary army is probably the biggest "private army" lead by a single individual on the whole planet. No kingdom, country and even government by definition can claim that. Even Robin was drawn to this figure and wanted to meet someone with that kind of flare. Wait, did I say "Robin" when we were talking about Poneglyphs?
Not only did Robin meet Dragon but she is a fundamental part of the revolution or at least that is how Dragon sees it. The Ohara incident seemed to have sparked the rebellion in the first place because Robin is called the "flame of revolution". The reason why the revolution was sparked was the extinction of a nation that hunted down the people who researched Poneglyphs. The pattern becomes clearer, especially because Dragon searched for the one person who can decipher Poneglyphs for over ten years. The connections are there but there are more long-term clues that we couldn't appreciate at the time.
Dragon ignores the question when Koala inquires if Dragon knows about "Drunken Iron Ore". Why is this important? Well, the material is almost certainly what Poneglyphs are made of. Koala says that only a small number of countries can produce weapons with it and Wano is known for its blacksmiths and of course the creation of the Poneglyphs.
Seeing now in context Dragon's reaction it becomes quite a big deal. If he wouldn't know about Drunken Iron Ogre he could have just said "no" but he didn't address the question at all. Meaning there is a deeper secret behind it that can't even be hinted on or it would give too much away. Another hint is extremely subtle and requires some follow-up questions to properly conclude.
Dragon is planning to meet Luffy one day. Why though? He obviously cares for his son and is pretending to not know him even when he obtained a 300 million belli. So he wants to protect his son by not tarnishing his name with his and wants to make sure Luffy can fulfill whatever goal he has, even if its a 99% death scenario like the road to become Pirate King? Safety is not a concern for Dragon but the mission itself is what is important to him. Today the story advanced, why doesn't he now? Luffy's name is beyond repair, so it wouldn't hurt him in the slightest if Dragon decides to show up but he doesn't. The reason is stated, Dragon believes that they will eventually meet one day but why? Maybe his first visit to Logue Town gives us more insight into his mission.
Dragon was on Roger's death day in Logue Town, that seems almost like a ritual to me and that he returns every year, showing that Roger is extremely important to him and his goals. The chapter title is "the legend has begun" and Dragon remembers Roger's words in his mind and replies to them thinking if the sentiment applies to Luffy. The response is "A pirate? That's fine, too..." and he grins right before he meets his son. Dragon knows Luffy might be on the path to fulfill the "will of D" (Roger's quote = will of D) and is making the connection between Luffy and Roger himself. Now we can answer why Dragon knows that his son will meet him one day. The path of finding Raftel and becoming Pirate King requires one to find all Road Poneglyphs.
One has been missing and for some reason Nekomamushi and Inuarashi don't know where it is, even though Roger has discovered it to find Raftel. However Roger found the last Poneglyph and it doesn't seem to be related to his journey directly. It is a special case that even the Dukes don't really understand or they would have just told us who had it last. We know that Dragon was present at Roger's execution connecting him again the man who found the last Poneglyph and a passing of the knowledge/will seems therefore plausible. Especially if we take into consideration that Dragon doesn't intend to meet Luffy on his own but still expects that they will meet one day. Once this meeting occurs the story progressed so far that its time to know about Dragon's journey and mission, which we will be talking about in the next chapter. There are two more points that I want to discuss. The first is the reasoning behind Dragon's tattoo.
Like prior stated, we can ignore the lines and have to focus on the 3 red cubes that are visible on his face. One could be hidden under the chin but if we keep the mystery of the Road Poneglyphs in mind this should sound familiar. Dragon has 3 Road Poneglyphs marked outside of his body and the 4th is missing, hence why it isn't shown. 3 are in the open and the last is hidden, sounds exactly like the kind of confirmation that we are looking for. The person who holds the 4th Road Poneglyph has the mystery of them marked on them in exchange for the hidden knowledge. Or if we go less extreme(which we don't have to, Poneglyphs are infused with will and hence why Roger could use the will of the Kozuki descendant to read and write in the Ancient language but that is for next time), out of duty for the cause Dragon has inked a visible tattoo on his face for the world to see but only the knowing will understand its meaning. Only someone who is on the journey to find all Road Poneglyphs and Raftel will know to seek the man. This is especially true for people who are able to gather 3 and are looking for the last one and see this famous person in the newspaper. They will get the meaning and the intention behind it. Let's finish this theory with a bang. You won't believe which cover page was shown when the Road Poneglyphs were introduced for the very first time.
I can't even remember a single cover page of Dragon prior to this point and he makes an appearance(correct me if I am wrong, it wouldn't break the case but would just make it even more stunning) when the "new" Road Poneglyphs come into play? This is not only suspicious but a giant red flag that Dragon is connected to the Road Poneglyphs. His tattoo symbolizes the mystery of the Road Poneglyphs and that ohe holds the missing one. To find Raftel one must meet the keeper of the last Road Poneglyph, the dragon that guards it.
This is it for today and I hope you enjoyed it. I will experiment on a more short and concise format by splitting up my theories into chapters. At this point, I see me posting theories with a huge scope as a big blunder and should make them more digestible. Thanks for reading and I am going to see you next time when we will be talking about...