What letter is 'missing' in our alphabet?

Obviously, none is missing, but for the sake of fun (or for your sake, because you really think there should be an extra letter) let us assume there could be twenty seven instead of twenty six letters in our alphabet (for the question I use the Roman alphabet).
Maybe a contraction of two letters that we use regularly, or a new letter alltogether?
Since there doesn’t seem to be the ability to write your new letter down here, try to explain it; both how it sounds and what it should look like.