An interesting topic is to study how to minimise data for the Internet of Things (IoT) for a certain connectivity of sensors in autonomous parking of motor vehicles. What type of wireless connectivity would require minimal data usage, for example.
People have suggested very specific topics on wireless communications. But I think for you the best thing is that to obtain a general understanding of the sub fields of wireless communication. Some examples are:
5G (which itself covers a lot of sub categories)
MIMO Communication (massive MIMO, etc.)
millimeter Wave Communications
Device-to-device and machine-to-machine communications, IoT
Energy harvesting for wireless communications
Heterogeneous and small-cell networks
Localization and navigation techniques
Ad-hoc and Sensor Networks
Wireless Security
Distributed, relay assisted, and cooperative communications
Cross-layer design and Resource Allocation
You can read papers and check conference tracks (e.g., http://globecom2017.ieee-globecom.org/content/call-symposium-papers), to have a sense.
Then based on your knowledge, time, and resources choose something relevant, or ask more focused questions. Like, what are the best existing research direction on massive MIMO systems?