I thought that Captain Phasma was a very underutilized villain, especially in The Last Jedi. She doesn’t really get to do much, gets unceremoniously thrown in a trash compactor off-screen in The Force Awakens, and loses to Finn in her only fight scene.
There were many things the writers could’ve done with her, like having her hunt down Finn and Rose on Canto Bight as they are looking for the code-breaker.