Totally separating mechanics from thematics or vice-versa is a fool's errand, but that doesn't mean they're exactly equal in importance. So, perhaps we can come up with a principle that reflects this. Something like:
Function follows form: The nature of a thing should guide you on how it works.
This is at best a rough draft hypothetical--as I have said many, many times, these things would need to be tested, a LOT, and almost certainly repeatedly revised. But this is the kind of thing I would expect as a "Principle" for D&D (sorry for making comparisons to DW, hope I haven't crossed your invisible line of "morph[ing D&D] into something it's not.") It would require elaboration, just as Principles in PbtA games do, because just SAYING it is not as helpful as actually going over it. The pithy phrase (in this case, "function follows form") is just there to make it easier to remember the guidance.
By comparison, an "Agenda"-equivalent is something that needs to be extremely high level--something that binds in basically all cases, less "guidance" and more "you need to be doing this for it to count as D&D at all." I'm afraid I'm drawing a blank for something that would work as a D&D "Agenda" that isn't simply a rephrase of one of the DW ones.