If I could fix anything, it would be IPv6 itself. The biggest thing preventing it's widespread adoption is it's complicated nature.
An IPv4 with an extra octet or two would have seen complete adoption years ago.
No new/subsummed functionality. Stick with ARP and DHCP, etc.
A better / more actionable upgrade path and plan to get things working so most devices could go v4.1 only quicker, etc.
IPv6 tried to solve too many problems at once. We should've just focused on solving the address exhaustion issue. Instead, we have a very slow roll out of IPv6, and awful stuff like CG-NAT taking permanent hold.
ipv4 is a mess in many ways, ARP for instance, is a disaster once you reach a certain scale and many of the ipv4 header fields are unneeded leading to inefficiencies. multicast on ipv4 is a mess, rfc1918 is a neat idea but ipv6 fixes it in a far better way and gets rid of NAT in the process. (and no, NAT does not increase security, ipv6 still has firewalls !)
in my opinion, ipv6 is the far simpler protocol, it is the migration from ipv4 to ipv6 which is resulting in all this complexity, but it is not the fault of ipv4.
and before people ask why not just extend the address space... that only solves half of the problems ipv4 has and not to mention, results in having the same dual stack situation as we have right now. extending address space is simply not possible in a backwards compatible way. and if we have to break compatibility, we might aswell fix all the other issues with ipv4.
Also, forward-compatible address extension is something which is simply not possible with routing protocols in place today, and i highly doubt it will be a possibility in the near or far future considering the enroumous coordination effort required between Core players on the Default-free zone and internet at large.