Visualization in machine learning

I am extremely interested in visualization in machine learning. I have read some scientific papers about this topic(like the visualization of real estate data) and personally think that this topic is quite satured by scientific research. Besides that I would like to contribute to this field in my free time and I am looking for research questions.

Therefore, my question is:

Which questions are still open in visualization in machine learning? What are interesting subfield in visualization?

I really would appreciate your hints about papers/research articles?

Best regards and much appreciation for your future answers!