It's hard to say how the majority of the fanbase will feel. Are we talking just the English speaking side of the fandom, or worldwide? Casual fans or die-hards? And there always will be loud voices on both sides shouting everyone else down.
But, I do feel like there's one aspect of the story that will probably make at least the USA side of the fandom VERY upset. And that's the Celestial Dragons.
Because I do feel like we're going to end up having a sad backstory for the Celestial Dragons. Why this group originally hid themselves away from everyone else. I feel as though Oda will try to make us sympathize with the Celestial Dragons, at least to some extent. And that is going to enrage a good portion of the English-side of the fandom.
Because, I'm pretty sure a good portion of them are expecting the Celestial Dragons to all die by the end of the story. And I just don't see Oda doing that. There's no way Oda has Luffy be okay with wiping out an entire people, no matter what they've done. Maybe if Imu or Blackbeard does it instead, as part of a plan to use them as fuel for some giant weapon, but even then Luffy and the Strawhats would try to save as many Celestial Dragons as they could. I feel like this one plot point is going to divide the English side of the fandom more than anything else.
And I feel like some of the things that might happen during the ending might come across as..."random" and strange to the more casual fans of One Piece? I mean, if we go to the Moon, or have an army of Dragons show up, or we find out the One Piece World is actually a giant Dyson Sphere...that might make sense to some of US, because we're pretty hardcore fans and have been paying EXTREMELY close attention to EVERY part of the story. But, I feel like some of the more casual fans would have forgotten some of these plot points after not really being focused on them for over a decade or maybe they never noticed them in the first place. So, I feel like One Piece's ending could be "confusing" for a lot of people who maybe just read the chapter once and then move on. I could see a lot of complaints coming from that.
And then there's the other elephant in the room. Oda's health. If Oda dies before he's able to complete the story, there's going to be a good chunk of the fandom who will be unhappy with the ending no matter what. Because we'll always be questioning on if it was really Oda's planned ending, or a ending that Shueisha/Shonen Jump thought would be more marketable. That's part of the reason why I'm hoping we get the ending sooner rather than later. Because, I don't want Oda to keel over doing this.
But, I don't want to be too much of a Negative Nellie. Because there is a pretty good chance that One Piece's ending could be something special. Oda has a plan. He's been seeding parts of that plan in plain sight for decades. And we're only now starting to notice some of the clues. If Oda is able to pull off a big reveal that changes how everyone looks at the series, then we might have a winner on our hands.
"Good" endings in Shonen manga don't happen that often, I'll admit. The track record is against Oda on this one. But, I'm not going to say it never happens, either.
Take a look at Jojo's Bizarre Adventure. Now, that's a bit of a special case, since every "arc" is basically it's own separate story. But, the vast majority of the endings in that series are actually pretty solid. With the possible exception of Stone Ocean, I'm not sure if people were upset about that one or not. But, every other ending I think has been pretty positively received.
Dragon Ball Z, I think, is another fair example. Sure, you have some people complaining about the Buu Saga. But, even the Buu Saga has some pretty memorable, crowd-pleasing moments. I don't hear too many people complain too loudly about DBZ's ending. Mind, that might be because the fights are really more the draw of DBZ, rather than the story itself. But, still, I think it ended just about as well as it could.
Fairy Tail is probably another example of at least being a "okay" ending. I don't think I've ever seen anyone complain about the ending. Maybe that's because no one really expected too much out of it. Especially since the author openly admitted to not planning anything out beforehand. But, I think it was a pretty solid ending, overall.
Assassination Classroom had a perfectly solid ending. I, personally, don't remember many people having much negativity about it at the time, at least.
InuYasha's ending was solid enough. At least they wrapped up all the plotlines. Again, it seems solid enough to me.
Yu-Gi-Oh, the original series at least, has a pretty solid ending. I actually really liked the Egypt Arc.
Shaman King...got it right on the second try. The "Kang Zeng Bang" ending is...maybe not traditionally "good" per se. But, it is VERY unique and memorable. I don't think there's an ending quite like this one in the rest of Shonen. That's worth something, at least.
So, while it doesn't happen often, I can name off a few series that had solid endings. At least, in my opinion. There is still hope that One Piece's endings could be one of the better ones.