Which course should I opt for in PG after completing the interior designing course?

You can do Masters in furniture design from NID. It is one of the best institute in India. The forms are available at NID site.

Hi, Vaishali, Greetings! A student can apply for the programme for M.Des in Fashion, interior, product design, and more, Students coming from any 10+2 or graduation discipline from a recognised university, or board with a minimum aggregate of 50%, can appear for any entrance or admission process or

There is a better option than NID for Master's Degree, you can just see to UCEED (an IIT Bombay program) admission will be just started: You can check the below mentioned link for more information: http://www.iitb.ac.in/uceed/2016/bdes.html . I hope this helps.