I was thinking about Jump top-3 selling places. 2019 we have granted that are going to be One Piece, Neverland and Kimetsu.
2020 is going to be pretty interesting:
-Hero Academia is going to come back strong for two reasons: S4 anime, and that the next volume (the main reason that I see it losing this year against 5toubun) is in December, which counts for 2020.
-Haikyuu is also going to come back strong with S4 anime.
-Neverland has in Winter the S2 anime. However, if it ends in early 2020, it maybe will fall positions in second half.
-Kimetsu is the big question. If it gets S2 in Spring (I think so), then it will continue like in 2019. Another incognite could be if Kimetsu is ending next year and when.
Basically 2020 will continue having Neverland and Kimetsu strong but with chances of them ending soon, and the return of Hero Academia and Haikyuu.
Now hyphotizing in a longer scenario aka 2021, I see Hero Academia and Haikyuu 100% recovering their top-3 places since Neverland would be over, and I'm pretty sure Kimetsu too. Only title that could maybe threat them would be maybe Jujutsu and depending of its anime.
I can't take Black Clover and Dr.Stone in consideration. This is a four-legged race. Also, I don't see Black Clover or Dr.Stone even being able to reach the Oricon top-10 in a 2021 situation without Neverland or Kimetsu. Still, is going to be an interesting scenario since it won't have Nanatsu, 5toubun or Shingeki neither, so I can't wait to see what new titles would be reaching the top-10 in the future.