I was reading on a site about the Zodiac Killer and how he used a basic substitution cipher, but instead of substituting english letters and characters he substituted symbols.

I was wondering, if you had a large enough subset of symbols to use for commonly repeating letters like E,T,S ... then by what means could someone decipher the cipher. This is also referring to the fact that no one has yet to decipher his last message. Obviously this type of system of cryptography would not withstand in modern computing.

I am more interested in other techniques that could be used, I also found this site '340 Cipher', which includes frequency counts.