1.You can take a look at Energy harvesting circuits with a comparison of different kind of antennas structures and the corresponding foam factor.
2.Filter Antenna are a very active topic these days.
3.Perhaps a more outbound topic but if you have knowledge and access to fabrication of all these antenna structures ,you can try and develop a neural network model for antenna designing using matlab or OpenCv.
Along with the above you can look into the 5G antenna design and for MIMO application.
Also, you can try to optimize the antenna design using some evolutionary computing tools.
Another interesting field of research with huge demand is the automated antenna design where you generally integrate the 3D EM Tool with little bit of coding in latest machine learning and evolutionary computing tools. Check the API of HFSS and integrate with MATLAB, complete the design and solve in HFSS. Later you fetch those results from HFSS in to matlab and run your objective function and repeat till the problem gets converged.
Autonomous wireless sensor node using RF energy harvesting or wireless power transfer is a good topic. In this application, antenna with filtering or differenctial antenna or confomal antenna or textile antenna are possble used.
You can also work on RFID chipless.
feel free to ask me if you write your proposal in these topics.