Dear friend

You can do your Ph.D. in several areas of ML and similar areas.

Basically, you can try to do in the field that synergies with other fields i.e. you need to do in several areas of different fields which are interrelated.

Such kind of cross field work will be more valuable and better recognized world wide.

First, the newer design of ML algorithms (most interesting).

Second, the way you perform the optimization of the algorithms.

Third, the robustness of the algorithms is the vital part i.e. how your algorithm is going to perform in noisy environment.

Fourth, the hybridization of algorithms is another part where people see a better way of developing more efficient algorithms. (called ensembles).

I have addressed the above issues in my latest publications. If interested you may look at them.

well there are some other issues besides the above mentioned ones.

You can start doing the literature survey and you will get the idea of what to do.

All the best for you Ph.D. work.

Best regards

Dr.Indrajit Mandal