The WSN with dynamic agents is relatively new topic in WSN. Secondly, as many cpuputing devices such as mobile, consist of multiple sensors, the gap between MANET and WSN has reduced and therefore most of the challanges related to MANET are also applicable to WSN.
WSN for medical health care application also have few additional challanges than the traditional WSN. Finally, Multimedia sensor network have QoS requirement on top of energy saving requiremtns.
No, this is a very old topic. There are hundreds of papers on indoor localization techniques. Frankly speaking, this field is very saturated and its quite difficult to do some extra ordinary work, unless you focus on the hardware aspects. If still interested, you can explore the usage of WSNs in healthcare and smart cities.
Data Collection using mobile sink, Sensor's Battery recharge using mobile Wireless Recharger, 3D Scenarios simulation, Energy Harvesting these are some hot research topics in WSN.