I believe that Franky is slowly turning the Thousand Sunny into the Ancient Weapon Pluton. I understand that he burned the Blueprints, but that doesn’t matter to someone like Franky.
He is considered to be one of the best shipwrights in the world alongside Iceburg. If you remember right before he burned the Blueprints, he flipped the pages extremely fast to confirm to CP-9 that they were real. If they were able to determine the authenticity just by a mere glance, then it would be nothing for Franky to memorize the entire plans within the time period that he had the blueprints.
Also the Sunny is made from the last of Adam Wood, the strongest wood in existence. It’s only fitting that the original Pluton was made from the same wood. The Gaon Cannon was already shown to be powerful enough to Decimate Duval’s hideout, and that was pre-timeskip before Franky gained all of the technological knowledge from Vegapunks lab…
Imagine the power of the upgraded Gaon Cannon.
Another thing that is interesting to note, is that Franky as a child was always building Warships, and his dream was to build the Ultimate Ship and sail the world on it. Now think about what the Ultimate ship is? It’s Pluton. I believe that by the end of the series, the Thousand Sunny will be the Pluton of this Era.
*Theory by GraphicNature