base65536, and look who the author is :-D

https://github.com/qntm/base65536

Base65536? https://github.com/qntm/base65536

But you need to make sure to use utf-16 or utf-32 instead of utf-8, or you may be worse off.

While a couple of people suggested Base65536, that encoding isn't particularly compact, and it can't be as elegant as 65536 would suggest because it has to dodge special cases in unicode.

It's almost always the case that either Base32768 is denser, or encodings with 2^17 or 2^20 characters are denser.