Ljubomir Jacić : lot many works are done in these domains and still going on and seems crowded? considering these skills, what are those specific top 10 challenges you would like to list out.
I question the question posed - especially the use of the word "need". I say that we shall not put every effort only in a few threads of research: history teaches us that free research will give back a lot of insight and progress that a rigid research structure cannot.
Dear Jai Deo Tiwari, I fully agree with Michael Patriksson in that a large amount of basic research needs to be done outside the mainstream topics. Quite often seemingly useless results later turned out to find exciting applications that no one could foresee at the time when the research was done.
Dear Jai Deo Tiwari, as an addition to my first answer, please keep in mind that not every researcher (especially in developing countries) has the resources and funding available to work on the "the top 10 global issues"!
In no particular order, I would say sustainable development (with relation to energy, water and pollution), Bioinformatics, Artificial intelligence and Data analysis. My response is leaning towards the Sciences but I can't help it, I'm a Life scientist. Lol.
In the field of Life Sciences, I will say research into improving sustainable energy and platform chemicals. Environmental protection and climate change is another area
Doing a PhD is an incredibly daunting task. Normally at least 3 years, there are some challenges that you are almost certainly going to have to face. In below link, we look at some of the biggest (and most common) problems that PhD students encounter.