Agile topics tend to be undertaken in M.Sc dissertation projects on Software Engineering methodologies. I am unclear what would qualify for a Ph.D dissertation and what kiind of studies would need to be conducted. There might be more about this in Computer Science Education research, or more-likely in institute projects such as those by SEI. Also, Tom Gilb might have guidance.
The suggestion by Mohammed Sabah is interesting, although one has to distinguish between the technology for blockchain ledgers and applications in use and what the security and threat models are. This is not an agile topic though. It is certainly one on cryptographic security and how the authenticity and integrity of a ledger are maintained. It is not clear what open problems remain at the technical and theoretical level. Do not be confused by the hype and magical thinking with regard to societal purposes. That's not a computer-science research topic as such, unless you are in position to investigate social impacts of technology and its conceptual difficulties for informed public understanding.