For an innovative idea regarding a topic for your PhD, you need to check the call of papers from different publishing groups like emerald, Tylor & Francis, science direct etc.
There you will find the most desirable topics that you can choose for your PhD research in Finance.
There are various types of taxation and in Finance they are imposed for a purpose. Look at a "Hallal" Tax. It is a tax on turnover. Look at Production taxtion. It is charged on Gross Profit, a mark-up on the cost price. Then look at Net profit taxation which allows for the deduction of certain costs like licences for Google head office charges for using a program or trade name.
Then go a little further down and look at the retained income after payments to the green factors.
Research is linking two or more constructs together to see if there is anything that is similar or universally different.
You are interested in Banking. Look at definitions and see if there is a similarity between two types of asset classification. Also what is the definition of income, cost? Look at Ontology, which is the way that people think and also Epistemology, which is how they arrange their knowledge. Is the way of thinking different or the same and is the way that people arrange their knowledge similar or is it just the same as if we are looking from the other side of the room.
I would suggest to investigate the main variables that condition the Return on equity of the islamic banking. I am doing a phd deep diving into the return on equity on spanish banks (and comparing to new competitors as fintechs, and assessing the impact of technology and new distribution models, etc.). If you decide to go through this, please let me know and we can make any kind of collaboration or maybe crossed investigation about return on equity of different banking industries.
I would suggest to take any title related to ( Islamic banking performance e.g ROA, ROE and NIM .
OR ( the effect of financial risk on performance ,here you have many risks like credit risk, liquidity risk , interest rate risk, operational risk ........