One Piece's Emperors might be the hardest to take down, as it takes a lot of potential to get to this position. While some people consider these individuals the strongest, some consider them the most influential. All in all, the Emperors are the strongest pirates of the sea, whom no one could face alone. However, one character took down three Emperors at a time, and fans might not remember him.
Introduced in the Impel Down Arc, Megellan was the only character who defeated three future Emperors (Luffy, Buggy, and Blackbeard) without breaking a sweat. While some fans might consider this a fluke, given the urgency of the situation where he defeated them, the One Piece character's achievement might make his return in a post-timeskip battle the most anticipated.
Disclaimer: This article contains potential spoilers from the manga series and expresses the author's opinion.
Exploring why Magellan might be the most anticipated One Piece character who should return to the frontlines

Magellan was the central antagonist of the Impel Down Arc and the warden of the Impel Down Prison. He was a moody person, and his mood fluctuated according to the situation he was in. As given from his role, Magellan shared the same hatred for pirates as did the World Government.
During the arc, Magellan had a close encounter with Monkey D Luffy, who was trying to reach his brother, Ace, who was held in the prison's Level 6. Eventually, these two fought in a heated battle, during which Magellan utilized his overpowered Devil Fruit.
Magellan ate the Doku-Doku no mi (the Venom-Venom devil fruit), allowing him to create and control venom. The potency of the venom varied from lightly poisonous to deadly, making him a menace to any devil fruit user or human. Against Luffy, Magellan was eventually forced to use a deadly poison, drowning the Straw Hat Pirates' captain in it.

This left Luffy in a critical situation, thus defeated by Magellan. During this fight, Buggy and Mr 3 tried escaping but were caught by Hannyabal. So, Magellan's fear indirectly led to the defeat of Buggy, too.
After Luffy was defeated, Magellan crossed paths with the Blackbeard Pirates. However, this fight didn't go on for long as Magellan used his poison to overpower the entire pirate crew, including their captain, Marshall D Teech. Interestingly enough, in the future, Luffy, Blackbeard, and Buggy became Emperors, alongside Shanks.

However, as already mentioned, these three were still defeated by a single character whom the fandom doesn't even consider overpowered or up to par with the strongest people on the Grand Line. So, what would happen if Magellan returned to the frontlines post-timeskip?
At the climax of Egghead Arc, during Vegapunk's reveal of the 'truth of the world,' One Piece brought its focus back on Megallan since his debut in the Impel Down Arc. However, his return to the battle on Elbaph Island has yet to happen, and this might be a perfect way to showcase the growth of Luffy since his first encounter with Magellan.
Final Thoughts
The emphasis on Magellan's return at this point in the story is much needed, given how One Piece is introducing the strongest people, like the Holy Knights, who could defeat the Straw Hat Pirates. Magellan, who might still have the same hatred for Luffy, could be a menace to the Straw Hat Pirates. However, things might turn out differently this time.
Related Links
- The Elbaph Arc could lead directly into One Piece's conclusion
- One Piece's Elbaph arc will be Oda at his best, and his own words prove it
- Akainu's past will set the course for One Piece's Elbaph Arc