I'd love to see Japan but I'm not a fan of dedicated high speed lines. Same reason I didn't buy Marseille-Avignon.
I have liked your suggestion for the simple fact of being a new country. We need more variety of countries in this game and nothing like Japan, Italy, Spain ...... that have incredible potential, especially Spain, which is the second country with the most km of high-speed roads in all of Europe.
You can find specific Shinkansen routes to upvote in the Japanese routes thread linked in my signature below.
Would vote for anything that include N700 because It has such a unique cab interior and would be fantastic to play with!