The first go-to weakness when talking about substitution ciphers is frequency analysis. It assumes that there's a simple 1 to 1 character mapping between the plain text and the cipher text.
Could a frequency analysis be thwarted if this were a 1 to Many mapping?
For example, what if the input text contained say 30 characters: 26 letters, space, period, question mark, and exclamation mark. Enough for most communication needs between two people. The output could be quite a bit larger, like the 95 printable characters.
E could be mapped to multiple output characters that appear at unusual frequencies in the cipher text that don't look like
E on a frequency analysis. This mapping could also map
space to 1 or more characters so you can't analyze word length or word frequency.
Every character could map to at least 3 output characters and knowing that some characters are already pretty rare they may not even need to be mapped to that many. Perhaps just 2 or even 1 that way other more frequently used characters could map to 5 or 6 cipher text characters.
Is there a different kind of analysis that could target a cipher like this?