As a candidate for the last (lit. dev.), "negative capability" seems to suggest itself, although it may be debated whether this concept is more rightly a theme/style than a literary device. I personally prefer the interpretation as the latter. If you have no other sources, take a look at the previous threads on negative capability in this forum, to get you started.
As to themes, you could round up the usual suspects, though it might not earn you any credits. Greek and Roman mythology was one of his favorite subjects, as was, of course, love. But don't write that. Perhaps something to distinguish him is the crossbreed of these two major themes, and his personal flavoring of the love theme, which typically carries elements of both virtuousness and melancholy (the latter usually a consequence of the former). At least, that is how I think Keats would have liked to see himself described, omitting mention of romances like "St. Agnes's Eve" and "Isabella" (which he rather criticized later as immature and frivolous works).
As for styles, I don't know that Keats distinguished himself in any particular one, although his struggle (through the odes) to perfect a literary form superior to the sonnet, which he felt was flawed and inadequate, is perhaps noteworthy.
Just some of my own personal reflections; you must, of course, do the real research yourself

Good luck!