I think that Chitwan is the best option if the capital city has to be changed. Here are some advantages.

1.Centrally located in between eastern and western part of Nepal. And also located in between hilly and terai region.

2.  Plain extensive  land and abundant source of water. It can hold the huge population of capital.

3. Availability of Physical infrastructure like, hospital , college,transportation, power, communication etc (Bharatpur Medical College,  Rampur Campus,Cancer Hospital, Mahendra Highway etc)

4. It's the model district where people migrated from all 75 districts in the past, so got the nick name 76  district of Nepal. Wide diversity of people with different caste,culture and ethnicity.

5. Easy to interact with the Indian authorities in the future because  South Chitwan has a border with India

6. District of intellectual people and leaders.Every revolution has been emerged from Chitwan in the past

7. District of natural biodiversity like Chitwan national park, religious place like Devghat, Narayani River,recreational place like Nagar Ban, Maula Kaalika Temple etc.

8. Industrial and Business Center  Tandi Bazaar and Narayangadh

9.Agricultural hub for Poultry and cash crops.

10.Traditional and welcoming people of Chitwan.

So, why not Chitwan guys?