Further Research Suggestions

Signal strength fluctuation in BLE is an area which may benefit from further research. Wang, et al. mentioned short term fluctuations in BLE signal strength, but that long term signal strength is stable Wang 2016. Determining a minimum sample period for obtaining an accurate signal strength would be helpful for real-time positioning calculations.
Another area of research would be fingerprinting accuracy as it relates to environment complexity. As the environment becomes more complex, i.e. an office building with many smaller rooms, it then follows that the fingerprint signatures will be more unique, and most likely more accurate. Whereas, an open environment, such as an airport, will produce fewer unique signatures and most likely be less accurate. 
Standardizing the requirements of researcher's reports would be very beneficial for further research as well.  The lack of data in reports, such as transmission power, beacon spacing, and frequency, make comparing methods equally very challenging.  In order for beneficial long term research to be done, some sort of standardization would be very helpful.