Mostly removed (e.g. removing fragmentation). If you're making an incompatible protocol, might as well take the chance to remove the cruft.
> Add an extra octet to IPv4, call it IPv5. Done.
Have fun debugging nondeterministic routing loops lol.
It's beyond too late to change anything, but I imagine if IPv6 had started with a sensible rollout plan and worked backward from there, it would look somewhat different and it would have taken a lot less time to take hold.
The Fragment header is used by an IPv6 source to send a packet larger than would fit in the path MTU to its destination. (Note: unlike IPv4, fragmentation in IPv6 is performed only by source nodes, not by routers along a packet's delivery path -- see Section 5.)
Also is it mathematically unique not to cause any potential conflicts.
Much akin to your idea but mostly supported already