To elaborate on the NLA design concerns that motivate this a bit more:

NLA has a big problem that if not all actions keyframe all properties, evaluation result may depend not only on the current frame, but on the sequence of frame changes that led to it, because properties may be updated at some points in the timeline, but not others. This has awful effects on usability.

The well known workaround for this is to create a rest pose action that includes all properties that you want to use, and put it at the bottom of the stack. The obvious downside is that this is very error prone.

The only easy and reasonable solution I see to this is to basically make the workaround automatic, using the default values of properties instead of an action. With this approach, property defaults in rigs can in a way be considered to constitute a part of the rest pose.

However this means that all animatable properties now must have reasonable defaults. For transform properties this is already the case, and other RNA properties can be fixed in RNA where needed, but custom properties defined by the user through UI need a new UI mechanism to specify appropriate custom defaults.