Hey Nishtha, North East is amazing to visit, have been there during end of 2020 and start of 2021. To be frank, i was tired of commuting the distance. You can break down the states differently, i have been to Cherrapunji and Sikkim.. Cherrapunji was gorgeous, you can plan that :)
You can plan for northeastern state in December as December end and January starting is the best time to visit north east.
Below 2 Itenary you can explore. 1. Guwahati- Tezpur- Bhalukpong - Bomdila - Dhirang - Tawang 2. Guwahati - Shilling ( From shilling you can explore lot of hidden gems in Meghalaya) Tawang road trip is one of the best road trip you can experience.