Zoro suffering a major defeat in the upcoming future is inevitable, but the question remains as when it will happen. Now the reason why I say this is inevitable, is because this is the route Oda has always chose to use as the driving force to make Zoro want to become stronger. We first saw this with Mihawk, and then later saw this with Kuma, and then finally saw it with Kizaru. It's always been a common theme to showcase Zoro have a relatively easy time throughout the adventure, before Oda decides to drop the proverbial brick wall into Zoro's path, which causes him to want to become stronger.
Zoro has yet to even come close to that brick wall throughout the New World, and has by far had it the easiest he's ever had throughout the entire series. To me this is showcasing that Oda is gearing him up for a serious rude awakening. He's letting Zoro overconfident and cocky nature which was heavily prevalent in the beginning of the series, rear its ugly face more so now than ever.
With that being said, Oda has only dropped that brick wall in Zoro's face one time in Baratie with Mihawk, and every other time it came at the end of the arc. Zoro has never suffered a defeat at the hands of an opponent, picked himself back up in the same arc, and then rematched the opponent and defeated them.
So the question remains, is what would make Wano different than previous arcs in order for Oda to drop that Brick wall in Zoro's face, mid arc. The answer is simple. Because this is Wano, the Country of Swordsmen. The absolute perfect place for Zoro to train and become stronger, but he'll never do that unless he is given a reason to.
But now there is another issue with this, and that is the fact that Wano seems to be entering into the final stages of the arc, and will be soon approaching it's climax.... Or are we? You have to realize that we are still in the middle of Act 3 of the Wano arc. The act, where in most traditional Kabuki plays, is almost always destined to end in tragedy. The way Oda has set this arc up, has all signs pointing to the raid on Onigashima, ending in utter failure. Even back in the beginning of the arc, we were explicitly given a one month timeline via Hawkins prediction, and yet here we are, only two weeks into Wano. That means we should still have at least two weeks of Wano left.
So what will the Straw Hats and the alliance be doing for those two weeks? The obvious answer would be re-evaluating everything that went wrong, and working on getting themselves stronger in order to prepare for the inevitable rematch with the Beast Mom pirate alliance. And like I said before, Zoro will not push himself to get stronger, unless he suffers a crushing defeat. It's the only way his character progression of wanting to get stronger has been built up over the course of the series.
Now with all that being said, I will say this. If this Onigashima raid is truly meant to be the end game for the Wano arc then ZORO WILL NOT LOSE. BUT, if a major tragedy is meant to happen during act three, and the Straw Hat alliance is meant to suffer an utterly crushing defeat, then Zoro will be ranked among those defeated, and it will drive him to want to become stronger, and it will most likely come at the hands of a strong Samurai (Perhaps Kyoshiro) who teaches him the ways of the Swordsmen of Wano.
