I don't think Phasma was meant to be anything other than "the cool looking character who becomes a popular action figure" like Boba Fett or Darth Maul. Indeed, the actual intent in TFA was she got stuck in the garbage compactor and was killed when Starkiller went boom. It's only because Rian Johnson and Gwendoline Christie are friends that she was brought back in TLJ. And though she does end up getting killed there, there was a deleted scene establishing she may not be dead after all, though TROS clearly didn't follow up on that.