Beyond Splatoon mania, a Breath of the Wild sequel.. is certainly going to propel the franchise to new heights in Japan, not as much as Splatoon but still it should have no problems to eventually join the 4 million club, something no one would have predicted for Zelda in Japan.
This is a bold prediction, considering that Breath of the Wild (the best-selling Zelda to date in Japan) has sold almost 1.9 million physical units, and even adding digital sales I have doubts that they lead to a total physical + digital over 3 million.
Also, never on a Nintendo console did the second Zelda sell more than the first; at most Adventure of Link was less than 100,000 units from sales of the first Zelda (source: https://vgsales.fandom.com/wiki/The_Legend_of_Zelda ). It's highly unlikely that Breath of the Wild 2 will sell more than BOTW 1, both in Japan and around the world - not least because, even if it's as good as its predecessor, it won't have the cultural impact or the "system seller" potential of BOTW 1.
It sounds good when you say "For the People", but what you really want is... a stronger army than the Knights, and the evil power to control the people.
(Ramza Beoulve, Final Fantasy Tactics)