If Eternum were One Piece:
ET 0.1-0.3 would be like the East Blue Saga to Alabasta. Character-focused, adventure-focused, wide open limitless world where you make the best of friends you'll ever have. Epic resolutions to epic adventures. Stories about human emotions, and the struggle of human connections.
ET 0.4: Enies Lobby, since it's often called "peak One Piece". The heist in Eternum will remain as one of my most favorite experiences in all of gaming, ever. Just like in One Piece, it's kind of the arc that really feels like the stakes have SUDDENLY surged as our heroes challenge by far the biggest baddest element of the power structure up to this point. It's 100%, all-out, need to use every single resource at our disposal just to come out alive. The date with Annie is probably the most substantial one-on-one event we've had so far. It's probably the event that so far has felt the most like going on a lifechanging trip with your girlfriend and shit goes wrong.
ET 0.5: Skypiea and Thriller Bark arcs. Love'em or hate'em. Tons of crucial character development, extremely important arcs with a slower pace of "top-level" story progression.
ET 0.6-0.7: Sabaody to through the timeskip. The characters' arcs are being increasingly tied with the high level powers that be, and we get more and more of a picture of the overall structure of the world.
End of 0.7 feels like the timeskip itself, as things fundamentally change afterward. Just like in One Piece, the protagonist has experienced something all on his own, that doesn't involve any of his friends that the whole story's been about so far, and comes back being different.
ET 0.8: Fishman Island/Punk Hazard, with end state resembling that of Dressrosa's conclusion. Many mysteries have been solved, there's no more "anything could happen" endless adventure feel anymore, you feel like you can see the rails that the story needs to proceed on now. Luffy's gonna take out the four emperors one by one and fight Blackbeard for the Rio Poneglyph. Orion's gonna get the gems of doom one by one and face off with Thanatos. The protagonist is known across the world and is steadily amassing popular support.
Yes, neither story has concluded, so we don't know for sure. But to a certain extent, we see some rails for the foreseeable future that the story can't really deviate from anymore, and we'll possibly have to sacrifice character development for plot progression. One Piece has DEFINITELY been guilty of this. Does anyone actually give a shit anymore when Zoro faces off with whichever next dude he's gonna slice up with a couple big swings? There can definitely be more stuff that happens after our current rail stuff (like Luffy's world after finding Raftel/fighting Blackbeard/fighting a huge war could definitely have more stuff unexpectedly tacked on at the end, and that's certainly possible with Eternum too - maybe there's much more to be done after getting all 10 Gems of Doom, or something happens that invalidates that objective entirely.
