|Derek Jacobi in I, Claudius. (From the Guardian.)|
Claudius, one of the most scholarly emperors, thought that Latin needed new letters for sounds that it couldn't easily represent. (Think about how English has no single letter for the /x/ sound in Scottish "loch," German "Bach," or Hebrew "Pesach," and you have some sense of the issue that he was trying to solve.) The new letters looked like this:
They probably roughly corresponded to the sound "ps" or "bs" (like in "apse" or "absent"), what we would call a W (like in the word "wayward," or Latin "voverat"), and a much-diminished whisper of a letter H that's hard to describe. Claudius' letters were apparently used extensively in his own lifetime, but fell out of use after his death, leading one to suspect that people used them just to flatter him. ("Quick, the emperor's looking—let's use his funny new letters now...")
We normally think of the letters we use to spell a language as fixed, evolving, undesignable things, but at least a few alphabets were either intentionally designed or had major interventions staged to reform them. The Cyrillic alphabet used to spell Russian and many languages spoken in the former Soviet Union was introduced in the Middle Ages by Saints Cyril (thus the name) and Methodius. The hangul alphabet used for Korean was designed by a team of scholars commissioned by the fifteenth-century King Sejong to be a maximally simple alphabet, in the hopes that it would improve literacy by replacing the complicated hanja borrowed from Chinese. The great Sequoyah invented almost wholesale a syllabic alphabet for the Cherokee language which is probably responsible for the Cherokee language's strong survival into the present. And in the 20th century, Communist takeover frequently came accompanied by a desire for spelling reforms, usually in order to improve public literacy. The Russian Communists banished several letters from Cyrillic, and Mao introduced major simplifications to a number of Chinese characters and radicals. It takes a lot of political power to change how an entire language is spelled. And even a Roman emperor didn't have enough power to do it on his own.