Reserved Top Level DNS Names
For that matter there are the user assigned code elements of ISO 3166-1 alpha-2. OK they are country codes rather than ccTLDs but we can be pretty certain that they won't get used as ccTLDs as it would cause too much confusion.
TL;DR version: the following two letter combinations can currently be used as non-conflicting TLDs and are highly unlikely to ever conflict in the future - AA, QM-QZ, XA-XZ, and ZZ(*).
(*) Long beards optional.