USA

If the NX is a home console it's a bettter idea to announce it in the West (USA or Europe, though USA is a bigger market for Nintendo) than in Japan, where home consoles are almost dead.

I would say for Japanese games to show it in Japan.

Worldwide online digital Nintendo Direct before a new Nintendo event in the fall probably in Japan that will include select stores outside of Japan getting early access demos similar to what they have done with E3 in the past.

At this point I think the NA market is more important to Nintendo than the Japanese market, just seems like the market that's far more willing to purchase consoles these days.

Maybe follow it up with a "Splatoon!" port or sequel announcement in Japan lol

US with a livestream event.

Everywhere guys, come on.

Probably in the US? 

It will be a digital presentation where they have the press at their own events where they will get "hands on" with the device so they can report about it. They did this once before with the ND/press day combo.

Europe. ;)