The true hidden treasure at Raftel is the realisation or "awakening" that the people are the true treasures. This means every character they met along their journey are treasures. Their loved ones are the treasure as well. This includes the characters that died for our main characters and treated our faves as their "treasures". So yes, our crew is the treasure as well.
I think this concept was touched on during Skypiea. What made Luffy and his crew pirates, was the fact they stole some meaningless gold, even though the people they saved were going to give them it all and more anyways. They knew they saved them, but they still stole it anyway.
The reason Roger laughed was because, he had to wait for Joy Boy to return for the World Government to show their true hand. Since Joy Boy has the power to free the people, once he returns the World Government would do everything they could to erase him for good.
This is Roger's true treasure as well, what he did by influencing everyone was also probably what the WG wanted anyway. They wanted to force Joy Boy to be born. The same exact way we saw towards the end of Wano Kaidou was trying to do as well. This is why Kaidou is set to revive at a later date. The Kaidou we see might be a completely different person.
Roger's treasure was made the moment he made his final words at his execution. He influenced his treasures to find the treasures to protect the treasures.