As someone who quite enjoyed most of Wano, I don't think much would need to change. Mostly just the ending. For whatever reason, it was rushed, and a lot of the multitude of tangents Oda set up in Acts 1 and 2 were not tied up well or at all. The scale isn't the problem because Oda going on tangents is basically the reason all of One Piece exists, especially pre-TS. And if you start changing too much, it just isn't the same arc is it?
The earliest thing I would change is to reveal O-Tama as a Kurozumi at the end of Act 1, but not to the characters. There's a lot of opportunity there for tension; What will they do if they find out? Why does she live in poverty when she's a relative of the Shogun?
Then in Act 2, I would make the secondary focus Robin getting the road poneglyph, revealing at the end of it when she finds it that Pluton is in Wano. This is again for the sake of tension (as many complained Arc 2 was flat/dragged out), and it also gives Sanji to like using his raid suit for something other than peeping. Probably reveal Sukiyaki here too.
Onigashima could be shorter, but otherwise the same apart from the fake-out scabbard deaths up to 1042 with Luffy losing. Don't immediately do the Nika reveal next chapter, but let the sense of defeat build. Instead of going downstairs, Kaido drops Onigashima on the capital with Momo holding it up just like Oden. Lots of great dialogue opportunities there with the fire festival lanterns rising up around them. Obviously Momo can't win, but instead he calls on Zou to start opening the borders.
Kaido, realising his fortress is already destroyed and the natural defenses of Wano are being destroyed, picks Onigashima back up and throws it at Zou. Law uses the last of his strength to move everyone to Zou and Kid uses the last of his to repel the magnetic core of Onigashima off-course and into the sea, on top of the WG fleet.
Kaido begins rushing towards Zou as Momo chases him. The strawhats can't find Luffy. Just as things seem most helpless, Jinbei thinks back to what Who's Who said about slaves praying to Nika, and he thinks of Fisher Tiger explaining the Sun tattoo: the slaves in Mariejois had prayed to the sun god to save them from the tyranny of the dragon's hoof.
It's then that we get the Nika reveal exactly as it happened, except maybe the Nika silhouette could be in front of Kaido's boro breath rather than the moon. Same great G5 fight with Kaido being defeated. Momo still decides to tell Zou to stop.
The post-arc should be more forward about why Yamato is going to stay, and resolve her need to be like Oden with Momonosuke's presence. Both feel like the other is more like Oden than themselves. After the "Kurozumi was born to burn" scene, have some reveal of Tama being a surviving Kurozumi start a mob like the old days under Sukiyaki. Sukiyaki stands in front of her and gets beat instead. Momo arrives and Tama is scared of the Shogun, but he shares his friendship with her, maybe he has brought red bean soup or a joke they had between each other.
The other thing is the smile fruits thing with Chopper. Only needs a page or two. Luffy's Nika (smile) fruit is an absolutely genius way to flip something that has been built up since Punk Hazard. Since we don't translate Nika that meaning hasn't clicked for most non-JP fans. Just have Chopper lament that he can't fix it and have the Ebisu residents tell him it's okay because now their laughter is real.
Personally I don't care about the Zoro's family stuff and I think he got just what he needed in Wano with Enma, the Kozaburo flashback and the King fight. A scene of him honouring Ryuma or taking Shusui back to his grave would've been nice but that's about it. His family history in contrast to Sanji is totally irrelevant because the core difference between them is that while Sanji is naturally capable but held back by a million different personal relationships and convictions, Zoro is tied down by nothing and is wholly dedicated to the hard work he has to put in to be the world's greatest swordsman. Zoro-Sanji agenda guys would ruin the story if they had their way and I enjoy every time Oda hurts their feelings.
None of this is to say any of us can write the story better than Oda. I saw no-one predict Gear 5 or anything similar. No-one understands the story like Oda, so only he can come up with the right ways to develop it. I'd never want to read anyone's two piece.