Wireless sensor network is a wide research area. Most of the researchers work on Link layer concepts such as MAC protocol for Wireless sensor network. In addition, security issues of Wireless sensor network are very important specially for Military applications. it needs more work and I think it's an open research area which you can focus on it. For example you can work on authentication methods which can be used for securing the communication between sensor nodes.

thanks a lot!!! for you valuable suggestions

sir recently i have gone through various papers on energy conservation mechanism on n/w layer like cluster head selection , techniques used like PEGASIS ,DEEC and many more but still i'm not able to make out what more can be done in these issues

and how to be more specific though i have published papers on these issues.

Energy Conservation is very important in all subjects of WSNs but you should focus only on parts which you want to improve. In addition, it's better to prepare a survey for your self and categorize algorithms in matter which you want to research about it. It's better to read some survey papers about Wireless sensor network. you can find a lot of them in IEEE or other digital library