Good Map, Bad Map
I think it is possible to have a fair federalism with the six states shown in the map above. And there is no need to give ethnic names to the states. Or perhaps a mix. Koshi, Bagmati, Gandaki, Karnali, Eastern Terai, Western Terai.
But it can not be a truncated Terai like in the map below where the easternmost and the westernmost parts are lost. And there is no arguing Chitwan is not part of the Terai.
Since half of the people live in the Terai, half the parliamentary seats have to be in the Terai. The directly elected president has to be a one person, one vote thing nationwide.