I would definitely be in favor of a complete overhaul of "can't be done while at war" - the default should be that everything can be done, and only the things that directly and immediately mess with a war (e.g. giving up provinces) should be disallowed. There's just not a good enough reason to prohibit things like changing rulers, expanding CoTs or the vast majority of other prohibited actions.
I would say even selling (non-threatened) provinces should be possible. You could just before the war declaration, so what changed fundamentally? For sure, the threat is more actual, but what if you are fighting a small nation on the other side of the world? Even if you are fighting a big nation, if the province is out of their reach, why couldn't you do whatever you want with it?
I would say, if CBs were to be changed so that you accumulate them (like in VIcky 2) and have to justify all your conquests instead of just blankly taking whatever you want, even leaving out your original CB, I would imagine a mechanic where provinces which are claimed in a war are those you cannot sell or interact with, since they are contested territory. Everything else, except if it's occupied or threatened, should be fully interactable with.
For a "threatened" province, I go back to either my (simple) version of "next to an ennemy army or occupation" or to
@jonjowett idea of multiple criterium for the same qualification.