I could see a new letter for the ‘th’ dipthong. But the list for each unique sound is enormous. And there are plenty of sounds that are used in other languages and missing in English. One of those sounds is the real sound of ‘f’ in the Japanese pronounciation of Fuji (I can't pronounce it, revenge for our 'l', LOL).
So I say we have enough letters as it is, government schools have enough trouble teaching just those.