There is only one game that I think has the potential to take the crown from New Horizon in Japan

Currently the Island Making Sim is heading towards 12 million units lifetime in the country and unprecedented doubling of what it the franchise managed on the 3DS. 

Yet, I think Splatoon 3 has the potential to surpass it...

- Splatoon 3 will be launching on a 20+ million user-base in Japan

- Splatoon 3 has had twice the amount of time for development & new ideas compared to Splatoon 2 which was launched to ensure a strong launch year in Japan.

Splatoon 2 was bit bear-bones at launch(this is a combination of design - rolling out features over the course of a couple of months to ensure high interest, and lack of development time to flash out everything). The game's last big update was Octo-expansion an year after launch, & they not really updating the PvE much beyond the first six months etc. 

Overall a lot of the issues with Splatoon 2 didn't end up hurting it's potential too much as it's a new franchise that managed 10 million sales, yet when we look at how MK8D, New Horizon, Smash, Breath of the Wild etc have blown up WW, there are still ways to see the franchise grow with the latest entry. The main thing is that instead of an 80 Meta

- I expect Splatoon 3 will be supported for a minimum of 2 years, below are some of my predictions for the game

• Single Player campaign comparable to Base Game + Octo-expansion 
• A greater number of modes and choices(2-16 player modes)
• PvE to actually get a lot more attention & at least 1 year of major updates
• A greater number of ranked modes but more choices for the players(being able to choose the online ranked mode would improve the game immeasurably)
• Having better community features will allow it to be even more viral
• A lot of the mobile app features will be available in-game including voice chat(you still won't have VC with Randoms)

- Global Testfires for the game will be a huge viral moment for the game

I believe these timed demos will create a huge hype online because Nintendo will put a lot marketing behind them, the userbase of the Switch when these huge events start will likely approach 100 million, meaning that it could be over 25 million users trying to login to Nintendo's online system at the same time.. 

- Critical reception at launch being better than either Splatoon 1 or Splatoon 2.

Those two games didn't have as much time to cook, and the soft-launch hurt their appeal among gaming critics(especially in the West). I think Nintendo will aim for better critical reception at launch by not holding back certain features as post-launch content. Having a "hard-launch" for Splatoon 3 will ensure that the game will be received better by critics(Splat 1 has 81 meta, while Splat 2 is at 83)

This game could be the straw that causes my household to buy a 4th Switch. She pretty much only plays in Splatfests but Spla2oon is the only game she actually plays occasionally. I haven't even see her play Kingdom Hearts or God of War in a couple years.... not that she was ever much of a gamer but ...

Splatoon 3 could potentially have the highest Opening of all time in japan, lifetime? BOLD AF

Way too many questions about what Nintendo does to make an accurate prediction. AC:NH came out in Switch's prime, whereas Splatoon 3 comes out in the last 1/3 of Switch's life, when people will start decreasing game purchases in anticipation of a generational leap with Switch 2 coming. But AC:NH could have amazing DLC content coming that enhances it's sales. Both games could have Switch 2 versions that boost sales as well. Way too many questions. If they released the same day in the same year then I'd say AC:NH wins easily, but that isn't the case.

I think Splatoon 3 could challenge its opening weekend but its way too early to say if it can challenge lifrtime.
As of now, Id say no.

Here we see Tbone in the wild licking his woulds as someone beats him to a bold Nintendo prediction.

Just kidding but after seeing you comment I just couldn't help myself.

Animal Crossing: New Horizons already is the best selling game ever in Japan, if you count Pokémon Yellow separately from Red/Green/Blue, at over 9.5 million and will end up over 11 million.

Splatoon 2 is still below 5 million.
There is no possible way in hell that Splatoon 3 sells over 10 million in Japan.

Yes, next year is very likely to be the Switch's peak in software sales in Japan, but ACNH launched at the perfect time and has been more popular before already. Both Wild World and New Leaf sold more than Splatoon 2.
Splatoon 3 would need to be a absolute masterpiece to sell more than 8 million unless it introduces some wild new ideas that raise the appeal even more.

I say 7 million is the ceiling.
Even Smash and Mario Kart won't have it easy to reach that mark.

Splatoon 3 will have strong numbers, but I have doubts it will even surpass Splatoon 2, let alone New Horizons. I would like to be proven wrong though

Kinda feel like unless they make some big changes to the formula. Splatoon 3 will start strong then taper off.

I initially thought you meant Horizon: Forbidden West and I was very confused.