They did have a proposal for 64bit...was ipv7.

They also had IPv9 with 20 byte addresses (160 bits) though some of that was consumed for common prefix announcing "this is a TUBA address". It was even something that was already supported by some hardware and software, as it was just dropping IP and replacing it with CLNP and transporting TCP and UDP over it (I think the most complex part was adapting ICMP-based tools).