Depending on the classes of nonlinear systems, there are several well-known control schemes for controlling strongly nonlinear processes, such as follows:
Feedback linearization
Integrator backstepping
Passivity-based conntrol
Control Lyapunov function
Sliding mode control
If you identify an unsolved control problem, then you may come up with a novel nonlinear control design. Of course, you can improve an existing nonlinear control scheme. Or, you can create a totally new way of analyzing the stability of nonlinear control systems.