Once in NE would you be travelling by flight or by road.
You can start from Meghalaya.Catch a direct flight to Shillong from Kolkata.2-3 days would be good enough for covering the must see places. Catch a cab to Guwahati (2 hours ride.shared cabs come at as low as rs 200.Visit Kamakhya temple during the day and take a cruise in Brhamaputra River in the eve.Next morning head to Kaziranga sanctuary (takes 3-4 hours max, start early morning to get the safari same day).Stay for the night.Next you can visit Majuli island, for which you need to go to Jorhat town(2 hours from Kaziranga. Explore the island on cycle, learn the Vaishnav culture and about the Satras there.From Jorhat you further go to Dimapur,Nagaland.
Flights to Kolkata are available from Dimpaur and Jorhat.So returning won't be an issue .
If you could specify your requirements,could suggest further.