Just go through the following paper. You will get the answer of all your questions my friend :)

In my opinion a hot topic is data plane evolution:

https://www.researchgate.net/publication/261959011_OpenState_programming_platform-independent_stateful_openflow_applications_inside_the_switch

and 

www.p4.org

Article OpenState: Programming Platform-independent Stateful OpenFlo...

@Bianchi, thanks for your respond. can you bringup the reason why data plane has new challenges in SDN and openflow and the relation to state full switching ?

Hi...

Open Research issues SDN

•Scalability:

•Control plane bottleneck.

•Single controller is not sufficient to manage large scale network.

•How many controllers are needed to support large scale network?

•When to scale down?

•Multi Controllers.

•Each controller is responsible to a subset of the network.

•Concern with synchronization and communication between controllers.

•How to slice the resources among controllers?

•Latency between controllers and switches.

•Less accurate decision?

•Slicing Resources (CPU, bandwidth, etc). 

- How to allocate resources to different controllers and users?

- Formulated to optimization and fairness problems.

•Using SDN to achieve more green DCN.

•No substantial works in this area.

•As 2015, few publications on this subject are published in IEEE ICC and IEEEE Globecom.

•Some software may provide measurement on power usage or  capability to turn on/off switches.

•NetFPGA, Mininet and OpenFlow?

::

 Future Research Directions

The future, however, will most likely use a combination of these (and perhaps other) models, standards, and implementations. Models such as I2RS are likely to be used to provide network programmability in those heterogeneous environments in which scalability  is the primary concern. The architectural models underlying I2RS envision layered, distributed control planes in which the control and data planes share fate.

Regards..

I think Traffic Classification, Network Monitoring and Standardization of Northbound API are good topics.

According to me Traffic Monitoring and Dynamic Resource Provisioning is a good topic to work upon.