Hi

Visit the ""CURRENT TRENDS IN NONLINEAR SYSTEMS AND

CONTROL"" book from L. Menini, L. Zaccarian and Ch. T. Abdallah, editors

you can find many aspects of your question there

be happy

If you are planning to write a dissertation, look for a new topic that bridges

areas that are under-represented in current journals. Nano-mechanics, biological

systems, discrete time systems, ...

In my field, which is quite active in getting attention is class of non-linear problems called Underactuated system. This class is observed through out the dynamics in nature, for example bipedal human walking,robot manipulators in space, supply chains etc. 

Of particular interests are the switching controllers, Non-linear Model Predictive Control,  Controller Synthesis, lie group based geometric controllers. 

Wang, T., Zhang, Y., Qiu, J., & Gao, H. (2015). Adaptive fuzzy backstepping control for a class of nonlinear systems with sampled and delayed measurements. Fuzzy Systems, IEEE Transactions on, 23(2), 302-312.

Liu, T., & Jiang, Z. P. (2015). Event-based control of nonlinear systems with partial state and output feedback. Automatica, 53, 10-22.

Chen, W., Sun, J., Chen, C., & Chen, J. (2015). Adaptive control of a class of nonlinear systems using multiple models with smooth controller. International Journal of Robust and Nonlinear Control, 25(6), 865-877.

Aranovskiy, S., Ortega, R., & Cisneros, R. (2015). Robust PI Passivity-based Control of Nonlinear Systems: Application to Port-Hamiltonian Systems and Temperature Regulation. arXiv preprint arXiv:1503.02935.