Roughly 20 years ago a network engineer I worked with said - "We took a look and IPv4 had everything we want and need but the address space". Why the committees didn't just made it 128 bit and be done with it is beyond me.
They basically did. All the other changes are minor cleanups like getting rid of fields nobody uses, and realigning header bytes.
Except SLAAC, but that wasn't part of the original design anyway. That was an accident when some major vendor implemented SLAAC before it implemented DHCPv6. The plan was to keep DHCP.
