My colleague @markusicu has proposed Base16k as an alternative to Base64 that is more compact when the resulting text is stored as UTF-16. This could be useful when storing byte arrays in JSON, for example.

https://sites.google.com/site/markusicu/unicode/base16k

If we're going to be opinionated enough to add Base64 to the standard library, then perhaps we should explore some competitors on their own merits.