Though it has been reworked, I think Diadem is dead again, and I think it will be even worse when Eureka is up.

Find a single Full Party is a miracle, and that is not enough for the Emergency Missions. So I was thinking... how about to "rework" it again and make the Diadem now as an openworldly area from Abalathia's Spine? So anyone could travel there at any time, (you should put an aetheryte there, of course, besides other things like Hunt Marks, sidequests maybe focused in exploration...).

The Emergency Missions spawn could be like the 6 elite FATEs: once each a maximum of 72 hours.