Not overly high tech but such a good fit for the culture and extremely convenient:

Self-filling, self-warming baths

Put the plug in during the day, press the button to fill the bath at the remote keypad in the kitchen. Baths fills and a little jingle announces that the bath is ready at the perfect temperature.

Medicare for all

Not Japan specifically, but I've got say I'm jealous as hell about the snack scene in east Asia.

I generally don't have a sweet tooth, and things like potato chips don't have that umami I like. I try to keep snacks around because I forget to eat, but nothing appeals to me. But man... all those pre-packaged tofu squares, various bits of marinated meat... that's my deal. There's one solid "Asian Mart" near me, I'll stock up a few months worth at a time.

Closest you get in the US is basically jerky/slim jims, which are great but expensive and kind of one note for flavor.

90s web design?

Kei trucks that are extremely functional and fuel efficient.

The U.S. won't ever get that because they are extremely functional and fuel efficient.

Hot coffee in a can that tastes great

If you select the wrong floor on an elevator, you can deactivate it by pressing the button again.

Plastic wrapping that's easy to open.