Including nearly all of NE, bar South Tibet that goes with China, would not greatly affect the demographics of BD. Remember whole of NE has 50 million people compared to 160 million for BD - out of this 50 million, 15 million are Muslims anyway,.
The massive land areas and moutainous landscape would allow BD to become a much more resource rich and scenic country.
Just imagine the tourism as BD properly develops the moutainous landscape. Millions of people from all over Asia would come flocking to BD then, earning many billions every year.
Abundant resources such as hydroelectricity would fuel BD's industrial growth.
BD and the NE are a natural economic match.