A game featuring only Japan -- whether just the areas they controlled during the era (Honshu, Shikoku, Kyushu, and some assorted smaller islands) or also the islands they've gained since (Okinawa, Hokkaido, and some assorted smaller ones) -- would possibly work, seeing as they didn't interact terribly much with the mainland during the era (they certainly didn't invade anything) and didn't get interacted with much by the mainland (with two major exceptions), even if it wasn't quite as isolationist as it was later on when they closed the country and stopped having it open.
However, the same cannot be said about the mainland (or a bunch of islands further south than Japan), seeing as those realms interacted quite a bit more with both each other and realms already on the map and as realms that are partially present on the current map held land further east and at least one country further east held land on the current map. If you created a game featuring those areas (or some subset thereof), you'd have the current situation flipped on its head for that game: "We need the steppes/Tibet/Tarim Basin/India in [the Far East game]!", which would be iterated on if those areas were added since they would be interacting with things further west... and I rather doubt it'd result in the requests for the Far East in CK3 would vanish completely with such a game.