Hi mpkd,
I would advise against choosing Plath's poetry as it is a set text, which suggests to the marker that you've failed to engage in the relevant in-depth research required to find a related text for your module.
Perhaps it would be a good idea to explore and understand how the thematic concerns underlying Plath's poetry are evident within other poetic anthologies, and to tie this in with your core text and module. This may allow you to find other sophisticated related material that shows the marker a higher degree of complexity within your choice of related texts.
Best of luck!