Hello,
There are many specialisation available with CSE in Mtech like as:

• Cloud computing

• Data mining

• Artificial intelligence

• Internet Of Things

• Machine Learning

• Image Processing

And many more also.
I will recommend you to go for AI or CLOUD COMPUTING as these have great scope in the future and rest depends on your interest.
Good luck

Mtech in Computer Science has various branches with one of best as Embedded systems engineering and software systems engineering which has seen humongous scope with advent of artificial intelligence and machines learning.
The specialization bring tremendous opportunities into roles like data scientist with handsome salary prospects and great learning.