If the extra stuff is mandatory for global reachability, again, it’s conceptually a mandatory part of the header, no matter where you actually put or what you call it.

> The central idea is that a IPv4x packet is still a globally routable IPv4 packet.

That's cool and all, but end-user edge routers are absolutely going to have to be updated to handle "IPv4x". Why? Because the entire point of IPvNext is to address address space exhaustion, their ISP will stop giving them IPv4 addresses.

This means that the ISP is also going to have to update significant parts of their systems to handle "IPv4x" packets, because they're going to have to handle customer site address management. The only thing that doesn't have to change is the easiest part of the system to get changed... the core routers and associated infrastructure.