The One Piece world that almost was. That's a lot to think about.
I am pretty glad the ideas of Sorcery, Peace-Mains and Morganeers didn't make it in the One Piece story. Sorcery is pretty much a Devil Fruit power without the weaknesses, you may even be able to learn all types of different sorcery so you have hundreds of different powers. The series would have probably turned out like Naruto where the all the ninjas fight like wizards conjuring everything from elemental spells to summons that do the fighting for you. Rather than making the most of what DF they're stuck with, everyone would be a little Blackbeard using the basics of multiple abilities.
Peace-Main and Morganeer puts a clear line between good and evil, something we're just not used to in this series. There a good pirates, there are bad pirates. There are good marines, there are bad marines. There are good kings, there are bad kings. There are (a few) good Celestial Dragons, there are bad Celestial Dragons. This would be a very awkward framework to build current the story on, but I guess we can imagine the ideas being fleshed out more like the Hero Academia manga. Would Peace-Mains help the Marines and vice versa? Would the 7 Warlords all be government-backed Peace-Mains? I assume so.
And then there's pirate Garp giving Luffy the Strawhat. I have to believe Luffy's promise is to surpass his grandpa. However, surpassing an old man probably past his prime is not very meaningful. What would Garp's connection to Gol D. Roger be here? Garp actually feels like an ordinary Peace-Main to me, not a walking legend or a future Yonko considering he's already old. But if his son and Luffy's father still ends up giving them a personal connection to Dragon, even here Garp should be anything but ordinary.
That was a pretty good special. I was hoping the anime added an extended fight between Luffy and Spiel, since this one-shot was lacking a good climax. This is a famous powerful sorcerer you've introduced here, of course you want to see what this guy is fully capable of in combat and still get outdone by Luffy. The one-shot that became canon with Galley works better since Galley is treated a joke in every sense, only a threat because he's underhanded and picks on the innocent. When he gets offed in a single attack, you can't say it wasn't fitting and he deserved something more.