Unfortunately, the public domain is fraught with uncertainty. There are legal arguments that you
can't voluntarily put something into the public domain, if you hold the copyright for it.
Part of the point of licenses like the GPL is to bypass that uncertainty by using the existing copyright laws to
force the provided content to be, and remain, as free as possible.
I had written up a bunch of ideas, but going over them piece-by-piece leads me to believe that a Creative Commons (CC) license might be entirely sufficient.
One of the key differences of the OGL vs other open licenses was that it allows you to only share
part of your copyrighted content, and reserve rights to other parts. That felt distinct enough that I thought it might be worth having its own license for.
But then I realized that what publishers have shared have generally been SRDs — curated collections of the original copyrighted work that include only and exactly what's being shared as open content. And ultimately, that's the only thing you have to share. So, rather than saying, "D&D is open content; this is the portion we will allow you to use.", you just say, "This curated document is being shared as open content. It happens to be the core rules that D&D uses."
In other words, you don't need the license to define the split in open content; you just need to separate that out into its own document, and share that document normally as CC content.
CC then has variations on the basic license, so that you can say that users of the content must share their remixes, may not share their remixes, or are free to share or not share their remixes; and do so in either a commercial or non-commercial fashion. That covers pretty much everything. All other issues, such as irrevocability or non-partisan stewardship are already handled.