All information I have read indicates the HND Terminal 1 extension will be domestic gates. Currently, all JL's domestic open gate spots (non-boarding bridge gates) which requires bus ride from the terminal are located in this new Terminal 1 extension area, which is at the north side of the current Terminal 1. I had my flights parked at this area on JL 737-800 domestic flights and J-Air E170/190 flights. What it looks like this new Terminal 1 extension will replace current open spot area used by JL, but the new Terminal 1 extension gates will be wide body capable. It looks like all JL international flights at HND will remain at Terminal3.

In 2021, the Ministry of Land, Infrastructure, Transport, and Tourism has made the statement about possibly extending HND Terminal 1 at the south end to the sideways and connect Terminal 1 (JL) and Terminal 2 (NH), where current south international area of Terminal 2 will be connected with Terminal 1 extension. This south extension area will be international capable gates. However, this seems to be an idea at this moment and no specific plan, such as initial survey, has not even started yet.

HND Terminal 1 north extension (website is in Japanese but a lot of drawings/pictures)