I personally can't see 1st Person RE being huge in Japan so aside from being on the Switch I think they would need to change the formula. 

Platform definitely plays a role but theres not some vast difference in PS4s userbase versus PS3s, yet RE6 almost tripled RE7 openning week.

Well, we don't know yet if the rumored Switch RE is first or third person, it could be a more traditional RE experience. That said, we don't even know for sure that it's a real thing so at this point it's all academic.

I need to know a lot more about this game in order to really picture it's full potential.
If it's a three year project that Capcom started in 2018, if it's a semi open world survival horror in third perspective I can see it catching on in Japan.

Obviously if it's a timed console-exclusive on the Switch, than we would see Nintendo probably treat it with a lot of marketing as it's definitely the highest profile horror game on the system thus far. Capcom has build an audience on the system with their ports and I can see that at minimum this type of project could sell 500K in Japan. Resident Evil is still a big IP and the recent Remake of Resident Evil 2 sold over 400K physical on the PS4.

I expect that in 2022 a lot of the traditional PS4 audience will have transitioned to the Switch, so I do think a potential high profile spin-off in the franchise can do very well in Japan and WW, at the moment without knowing much about the title I still think in Japan it won't have problems to outsell Resident Evil: Village, mainly by the virtue of being on the Switch.

However I would caution making any statements until it's revealed and we see how ambitious it is, if it follows the same path as Rise than we should definitely see Resident Evil see growth in Japan again

Information atm is spotty at best but the MH Rise leaker seems to think it's a continuation of the RE Revelations line, (though it probably won't be named as such as Capcom are moving away from numbering entries according to him) but with a bigger budget and a longer production cycle, close to MH Rise or RE Village.

This could indicate its a third person game like Revelations 1 and 2, though again we just don't know yet, and the leaker has stressed that they're not yet 100% positive it's actually happening or if something got lost in translation.