My prototype/thought experiment is called IPv40 a 40bit extension to IPv4.
IPv40 addresses are carried over Legacy networks using the IPv4 Options Field (Type 35)
Legacy routers ignore Option 35 and route based on the 32-bit destination (effectively forcing traffic to "Space 0". IPv40-aware routers parse Option 35 to switch Universes.
This works right now but as a software overlay not in hardware.
Just my programming/thought experiment which was pretty fun.
When solutions are pushed top down like IPv6 my spider sense tingles -- what problem is it solving? the answers are NOT 'to address address space limitations of IPv4' that is the marketing and if you challenge it you will be met with ad hominen attacks and emotional manipulations.