Ah, all caps! Eyes... burn...
Sorry, the all caps thing drives me a little nuts... please excuse my brief moment of off-topicness, I shall now answer your question.
So, you want to do your MA on symbols, metaphors, and themes in English poetry with reference to John Keats? That seems a bit broad. Would you be looking at all of Keats' poetry with possible references to other Romantic poets or would it be all Keats?
I think you could get away with just doing themes in English poetry. By looking at the theme, you could incorporate every literary device used to develop that theme, especially with reference to Keats. You could do just metaphors or symbols, but both of those really narrow what you could talk about. I like the idea of exploring common themes in Romantic poetry with reference to the works of John Keats and other prolific poets of the time period.
That's my suggestion, anyway.

Good luck!