One of the most under explored aspects of the Star Trek universe is the planet Betazed, home of what might be the only species of telepaths in the galaxy.
Fans of Star Trek: The Next Generation are most familiar with the Betazoids as that was the series they were most prevalent on. Deanna Troi was a half-Betazoid, half-human Starfleet officer serving under the command of Captain Jean-Luc Picard on the USS Enterprise-D. Because of her heritage, Deanna Troi was not a full telepath but did have empathic abilities. Her mother, Lwaxana, was a full telepath and had no problems using those abilities to torment Captain Picard.
Betazed has long been a prominent member of the Federation but little about its past, present or future has been explored. Outside of TNG, few other series have even included a Betazoid cast member, which is weird as telepaths can be extremely helpful crew members. It's definitely a race that deserves more screen time.