Those who have played Chrono Trigger / Cross would have known this New Game+ option which player replays the story mode but with levelled up and powered up characters, though the enemies and bosses remained at the same difficulty and HP.
I imagine we re-live the One Piece story up to pre TS using a current time Luffy (acquired Gear Fourth) but the main antagonists remained at the same level. Of course Luffy will now have a much easier fight. Lets revisit and reimagine them.
- Luffy could have just do the same thing Shanks did to the Lord of the Coast during his departure
- (fat) Alvida was never a tough fight, so I'll just skip
- the same could be said about Morgan, so another skip
- Buggy was not a very difficult fight, what Luffy could have done better is to anticipate Buggy's flying-hand-with-daggers attacks with his haki
- Kuro was all about speed, thus Luffy's mastery of kenbunshoku haki will ensure he can see Kuro's attacks incoming
- Krieg was all about tough outer defense (armor) and long distance attacks, Gear Second allows Luffy to instantaneously close the gap to point blank range and deliver a devastating punch via buso koka
- Arlong was brute strong, fast and high endurance, Luffy will be utilising kenbunshoku haki, Gear Second and buso koka to take him down
- Smoker was the first logia and untouchable opponent Luffy faced, thus busoshoku haki is the answer
- Crocodile was also logia but could be exploited via water, again buso haki is the answer to the challenge
- Enel's prime strength was not his logia, but his haki (mantra) so with Luffy acquired his own kenbunshoku haki, it is a battle of who's haki is superior and Gear Second may tip the edge significantly towards Luffy, buso may not be necessary because Luffy can already touch Enel
- Kuzan, though not the first logia encountered, was the first logia which coming in contact resulting a negative effect (frozen), thus combination of buso haki and koka will allow Luffy to simultaneously hurt Kuzan as well as negating the issue of freezing-upon-contact
- Lucci was all about endurance and the fight was mainly who can outlast the other, buso koka and superior kenbunshoku haki should allow Luffy to hit harder and better anticipating Lucci's attacks. Perhaps Gear Fourth can also apply well here
- Oars and Moriah will be taken out by Elephant Gun (buso koka + Gear Third) or Elephant Gatling (buso koka + Gear Third + Gear Second)
- PX4 will be dealt exactly the same as post-TS pacifista, nuff said
- Kuma should be the only opponent that may still pose the same threat, what is dangerous about this guy is his Nikyu Nikyu, perhaps having significant level of haki armor may negate or deactivate the application of his Nikyu Nikyu upon contact, I don't know just maybe
- Magellan's advantage was being untouchable due to poison effect upon contact, I think if not for Luffy acquired poison immunity, maybe he may still struggle the same despite using buso koka