It may depend on your advisor area of expertise. These are some suggestions:

1. Dynamic network defense. Adaptive network security is interesting in which you can investigate coupling different detection systems or develop a model to maximize security.

2. Insider threat ontology.

3. Game theory application in network security.

Good luck

auto detect and auto defend about new security threat in 2019 with Artificial intelligence science..

how to get verification human (or is valid human) with Artificial Intelligence (Adaptive self genetic Algorithm) focus of Trending Application on 2019

Good luck

Most trend work is the cybersecurity defence algorithm over 5G

IoT botnet detection

Florian Scheiber Wireless Sensor Network, distributed denial of service, intrusion detection system, intrusion prevention system.

https://www.quora.com/What-are-some-topics-on-network-security

https://howtowrite.customwritings.com/post/20-network-security-project-topics/

Network security, wireless sensor networks, wireless adhoc networks.

Cyber security, 5G networking

Security is so wide that it touches almost all other technologies. The simple reason is that most systems are designed to work first before looking at how secure it is, i.e. absence of security by design. So before you venture into security, what is it that your are interested in? The security of these vulnerable systems or attacks directed at these systems? Then what systems are we talking about? IoT, SDN, Cloud Computing, SCADA/ICS, Internet, 5G, etc. You can look at mechanisms, methods and paradigms....access controls, secure protocols, cryptography, malware, Advanced Persistent Threats (APT) etc.

So, in a nutshell, first choose and area of security (and this is largely dependent on the one supervising you ), then choose what exactly you wanna address in that area of security, and what methods/mechanisms/approaches you will use to address the challenge. I know it's a mouthful, right! :)