As the third strongest member of the Pirate King’s crew, Sanji will be one of the strongest characters in the the world at the end of the series. He probably will be the fastest fighter of the world and his final bounty will be about 1,800,000,000 Berries.
At the end of the series Zoro will be the Greatest Swordsman in the World and the right hand man of the Pirate King. In terms of power he will probably be Yonko level and his final bounty will be about 2,000,000,000 Berries.
Robin’s greatest threat is always from the World Government. At a young age, Robin acquired a bounty of 79,000,000 Berries for having the ability to read Poneglyphs, which told of the Void Century. In the current storyline Nico Robin is the only person alive who can read the language written on poneglyph tablets. The persistence of the World Government and their view on her being a threat would not cease even after a timeskip passed, as during the Raid on Onigashima, the World Government send numerous ships out to Wano, openly to annexing Wano under the World Government, but secretly having a second order for its agents arriving there with the capture of Robin, still seeing her as a threat to the Government and the secrets its trying to withold from the world. I could see Robin reaching something absurd like 2,500,000,000 Berries.
Currently, Luffy has gained a bounty of 3,000,000,000 Berries and has officially been declared as one of the Four Emperors by the World Government along with Buggy, replacing Kaido and Big Mom.
At the end of the series Luffy will take down Blackbeard and will become the Pirate King. For this reason, Luffy’s bounty will become the highest bounty ever seen in the series.
Since Gol D. Roger had a bounty of 5,564,800,000 Berries, Luffy’s final bounty will reach 5,600,000,000 (since 56 -> go-mu -> gomu = rubber) Berries.
Only after becoming the Pirate King, Luffy will take down the World Government.