I am about to write an essay with the subject, "Choose any one surviving Attic tragedy or comedy which contains one or more non-Greek characters: how is their representation as non-Greeks constructed, and what is the intended effect?"
I was hoping to study the character of the Thracian King in this play. Do you think there is sufficient scope to discuss this character for a 1,500 essay?
By "the Thracian King," I assume that you mean Tereus in his incarnation as a hoopoe. Expanding a discussion of Tereus into a 1,500 word essay would be a challenge and it seems to me this would require a rather detailed knowledge of the relationship between the Athenians and the Thracians around 414 BCE when Aristophanes brought Birds to the stage. To figure that out, one would need, at the very least, to look at Thucydides and another play of Aristophanes (Acharnians). Acharnians has some clearly negative caricatures of Thracians in the form of Odomantian mercenaries who appear early in the play and steal Dicaeopolis' lunch.
One would also need to look into the mythology behind Tereus, whose barbaric acts were carried out, interestingly enough, against a mythological Athenian woman (Philomela). The fact that the Athenians in Aristophanes' Birds would become allies with a mythical figure who committed a horrific outrage upon an Athenian woman is striking.
If I were studying barbarians in the Birds, I would also add the Triballian god who appears at the end of the play. The gibberish that this divinity speaks and his characterization as a complete imbecile makes an interesting comparison with Pseudoartabas from Acharnians.