Hello Mr.Mubeen. It depends on which sub-topic of marketing you are mostly interested in and what kind of a research you would like to conduct. You shold decide whether you will conduct a consumer research or business/manager/employee..etc research. And you should also decide whether you will conduct a quantitative or qualitative analysis. But as the contemporary era is the era of digitalisation and digital penetration, I would suggest topics related to technology, digitalisation, e-commerce.. etc. (but its just a suggestion, there are lots of sub-topics of marketing and multi-disciplines marketing works well with). I recommend you to download a 'marketing scales book'. There are two or three books where you can access via free book download websites, if you skim the marketing scales from these books you may hit on a good idea. Feasibility is also very important. I have seen lots of Ph.D. candidates who struggled to complete their thesis due to selecting a wrong methodology, inapplicable scales..etc.