They did say that they were as of 4.5 still thinking of which direction to take it. The developers stated that they preferred the Eureka method and that the old method being boring and unchalleging for them.

I think Diadem is likely tied to the rebuilding of Ishgard content. Or could tie in for both. Who knows?

Personally, I would prefer the Eureka method but can settle for the uninteresting ARR/HW way.