I would love a new standard stream for structured output alongside stdout and stderr. That seems like the lowest-friction way to get tooling migrated: programs could opt-in if they wanted but nothing would break if they didn't, and then once you had sufficient buy-in you could make a new shell that defaulted to piping this new stream and fall back to stdout if there was no data in the new one. But it would still need a lot of coordination to make it happen, since AIUI you would need to change libc and every other C runtime to pass this new stream fd to every program, and that would rile people up.