"But in IMC without outside visual reference, we're very bad at detecting a slow uncommanded roll, a gradual loss in altitude, deterioration of speed / energy, etc. All items which should be part of any pilot's basic instrument scan."
One of the "features" of the human component of our airborne system is that it adapts to its environment - meaning its skills, awareness, attention, behaviors are affected by its experiences. The modern automated flight deck has done wonders to performance, enabled operations, and so on, but it has also, unintentionally, changed routine pilot behavior. One of those behaviors, I believe, is the instrument scan - its quality of perception and its frequency. Given the evidence of extended airspeed degradation (over a period of minutes) demonstrated in the data of some accidents over the last several years.
I think we need some deliberate flight deck display design to both compensate for and correct this trend in pilot behavior. A cacaphony of new alerts, hand slaps, and training emphases, alone, are inadequate and unsustainable. Our flight deck displays systems must not only support instrument flight they must promote good airmanship. We need to embrace the characteristics of the human component, not complain about them, and we need to design with them in mind to foster the pilot behavior (habits, performance, awareness) we need.
Alerts may address crew awareness of a non-normal condition, but they do nothing to promote pilot awareness of the aircraft state under normal routine conditions. And the latter is the main problem.