Sand
Kerosene (just a few posts above this one) relates how the industry, seen as a worldwide entity or unit, has just now, with this tragic crash, pulled its head, ostrich-like, out of the sand. Indeed, the worldwide industry ostensibly lives under the banner of "Uniting Aviation" (literally, these hang inside ICAO world headquarters) - but why then does the industry appear to accept that so many States are only 60 percent compliant with safety standards, and many others not even that proficient?
1. What if access to the FD is restricted to a senior-level FA, someone trained and steeped in the safety culture of aviation? (Yeah, I get it, that such a senior-level person is not always present in CC, but that's like observing the world's .600 batting average on safety.) And give the FA the civil aviation equivalent to ROEs - Rules of Engagement. The experts can write them, but the general idea is to authorize what the FA can do relative to opening the door, and what not do. No crystal ball is 1000 percent transparent, but that is no reason not to try to address what is known and what can be anticipated.
2. Some cabin attendants already have the training and capacity - on some carriers at the least - to subdue unruly pax. This means the air carriers and regulators have found them - those who have such training and actually apply the plastic restraints in an incident - capable of abiding by the standards of aviation security and safety.
Under no circumstances, allow a recently-hired, relatively untrained, FA into the FD, given the availability of significantly more experienced, more trained, and hence more trusty and reliable, flight attendants.
3. Accelerate the Sky Marshall program and proliferate it widely. If you don't like FAs on the flight deck, and you are willing to have a Sky Marshall at all, then replace the FA with a trained aviation security and safety professional. (Plus ROEs, sure.)
4. Over time, and it will be a pretty long time surely, move certification standards so that having a lav inside the cockpit door becomes standard. And work in a 3rd crewmember - isn't it true that being seated on the FD was thought to be a great training exercise for ATCOs back in the days of the "fam" flight (familiarization)? - so, for the junior-cadre FOs who didn't learn to fly in the military or otherwise in a fully professional environment, would not such flights yield valuable training inputs?
Finally, unless and until the industry undergoes a financial, operational, and most of all, managerial-attitudes renaissance, humane and more pragmatic approaches to aviators with psychological concerns in their individual lives is about as likely as, well, as a return to those mythic days of Capt. Vern DeMerest aboard The Golden Argosy to Rome (apologies to Arthur Hailey, author of Airport).