During the last decade, a lot of research work have been focused on the IoT. However, several research issues remain. In particular regarding two aspects. Firstly, security issues constitute one of the most important challenge in IoT. In fact, gathered data will be highly private, any weakness in the used security protocols might threaten users privacy. In addition, resources scarcity in IoT makes existing security protocols hard to directly implement. In my point of view security is still an issue regarding IoT. Secondly, IoT will have to rely on a well defined architecture, that takes into account its different specificities such as heterogeneity, and scalability. The IETF, for instance, is taking the lead in proposing standard based protocols (e.g. 6LoWPAN, RPL, CoAP). Nevertheless, several challenges are still open.

IoT is still relatively new. We cant apply the schemes (energy efficiency, security, congestion control etc) of WSN and traditional internet to IoT because they were not designed while keep IoT in mind. Although, almost all of the areas require significant amount of research but i believe that security is one such area which require particular consideration. There is NO secure scheme for tackling Sybil attack, sinkhole, wormhole, dos and similar type of attacks. 

In first step you must have a good vision on IoT and infrastructures of that, then you could select a topic. In other hand main question is whats your skill and ability. I send a map of IOT for you, it can useful.