Encourage all aspiring airline pilots to read Ernest K Gann's FATE IS THE HUNTER. Ask them what they learned about the flight deck environment in Gann's day. Ask them to profile two captains that for Gann epitomised the respected, well-rounded aircraft commander. Ask them to instance an occasion where the skill, experience and resourcefulness of the skipper saved the day.
Select ten major accidents (say since 1979 Air New Zealand DC10 Mount Erebus disaster). Look up the analysis of those accidents. Select some for study and review where the interaction of the crew prior to landing or impact shows that the commander was highly competent.
Pick others were his input (or lack of) may have contributed or did contribute to the accident.
To my mind your brief outline of what you are wanting to do is way short on specifics.
I baulk too at the notion of 'nice and catchy definitions'.
One might prattle on ad nauseam about airmanship, for instance, but in will not be worth a tinker's curse if not rooted in long experience. The good commander is not born to his office. He is made. And not overnight. (Some recent horror stories of overseas 'qualified' applicants for check out in the sims at Tullamarine, Melbourne would give you the heeby jeebies.)
As a man experienced in A340 ops you should by now have a clear picture of what personal attributes you would like to see in guidelines to determine an airline pilot's command potential. So it begs the question, what do you hope to learn about this subject from others that you have not already sussed out for yourself.?
All of the above offered in the spirit of awareness, safety and economy.
May your own efforts be successfully directed along similar lines.
If you care to pay for the mail-out and have a questionairre put together I'd be happy to circulate it to a couple of hundred retired airline pilots if that would help your exercise.