Good afternoon, machine learning is a tool there is no hierarchy of research areas all contributions are valuable for each specific domain. I think you should determine your research interests. If you did not have an idea, maybe you can search previous projects reviewing papers, and datasets with common licenses for example on Kaggle to find out if a domain seems to be more interesting for you.

apply algorithm in ECG analysis. or x-ray image analysis. there can be various problems to do research on.

I suggest exploring articles from various fields that apply machine learning or deep learning to see if any topic catches your interest. Machine learning has diverse applications, and the major steps are similar across different fields. However, there might be some variations in the process based on the type of data you'll be working with. It's an enjoyable experience overall!