Granted, the “why I should care part” is more difficult.
Unfortunately, there is not. My guess is either that it has not yet been determined or decided, or it exists only in @marmarek’s head.
I think the closest thing we have is just keeping an eye on the Release 4.3 milestone to see which issues @marmarek adds to it over time.
There have been requests and attempts to have a public road map and related things:
However, these never really went anywhere. My guess as to the root cause is that @marmarek never had the time for them, and/or other things were always more important.
But all this talk about 4.3 is a bit puzzling to me. I thought we were talking about 4.2 testing.