Can you suggest me what to select for the phd and which subject is not developed enough in research and can be one of the first priorities after 3 or 4 years of research.
Nope! I can't tell you which of those is not sufficiently developed. If I knew, I would write a dissertation and earn a PhD in your field. The reason a PhD is awarded is that someone becomes an expert by doing the necessary reading. I know it is a lot of work, but you have to become an expert in all of these areas and keep researching, keep working until you become inspired to solve a problem or develop a subject that has been neglected by others in your field.
Here is a suggestion: Find 5 articles that interest you, articles that cover these subjects. They MUST be recent articles from the past 2 or 3 years. If you read some great articles, you will see what contributions were recently made, and you will get ideas about what is left to be done. It's just like if I visit my mom's house and see that she is cleaning her apartment: I look to see what she is has just done, what she is doing right now, and how I can help.
:-) I'm glad you are participating here in our forum!