Alphabets vary widely across the world, with some disagreement as to what constitutes a 'letter'. For example, most Spanish dictionaries treat 'ch' and 'll' as separate letters but, last year, the 'Real Academia Española' decided that they're not. (However ñ, which is effectively a double n, is still regarded as a letter in its own right).
The English alphabet has lost quite a few letters over the years [including thorn (þ), eth (ð), wynn (ƿ), yogh (ȝ), ash (æ), and ethel (œ)], whereas j and u have been added, with 'uu' later being replaced by 'w'.
It would be possible, for example, to replace 'th' with a single letter but, unles someone comes up with a particular reason for doing so, that's unlikely to happen. Perhaps though, as world languages intermingle more and more, we might find that there's a word, imported from (say) an African or Asian language which has no easy way of spelling it; somebody might then introduce a new letter for the awkward sound.
Chris