> This made me think... why are we using long, unwieldy base-16 hex strings at all? Why not use an alphabetic (non-numeric) base-46 string: 20 lowercase letters ([g-z]), 26 capital letters ([A-Z])? Then the new SHA256 hash strings end up being shorter than the old SHA1 strings, and there is no overlap with the [0-9a-f] range of the base-16 strings.

Are you sure? SHA1 hashes are 40 hex digits, and SHA256 hashes are 64 hex digits. But in base 46, 2^256-1 = 3ZS4A7V5Ki0LWg1f3Of06YNfgQXCA2P0Q6RACKhEIWQXe07, which is 47 base-46 digits long, so still longer than SHA1. (This is not your base 46, since it's using 0..9, A..Z, a..j.)

  import gmpy2
  gmpy2.digits(2**256-1, 46)
    -> '3ZS4A7V5Ki0LWg1f3Of06YNfgQXCA2P0Q6RACKhEIWQXe07'
  gmpy2.digits(gmpy2.mpz('f'*64, 16), 46) #same, but more clearly the "maximum" hash

edit: more code to try kelnos's proposed digits:

  gmpy_digits = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz'
  new_digits = 'ghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'
  ''.join(new_digits[gmpy_digits.index(d)] for d in gmpy2.digits(gmpy2.mpz('f'*64, 16), 46))
    -> 'jPIkqnLlAYgBMWhVjEVgmODVWGNsqiFgGmHqsAXuyMGNUgn'

I think hex strings are probably still better, as there's less ambiguity, and 47 characters isn't much shorter than 64 for practical purposes.