Mantis

In the comics, originally just an advisor for the Guardians, Mantis proved herself because of her empathic abilities, as well as because she is a very skilled martial artist (actually, one of the best in the whole universe). Granted, in the MCU, the Guardians already have such a team member "“ however, she could be very useful because of her ability to sense emotions and use them in order to manipulate friends or foes.

Sure enough, it is worth mentioning that, in recent times, in the comics, she is also a telepath. However, this could easily be considered just an extension of her empathic abilities. So more reasons for her to be in Guardians of the Galaxy 2.