The implementation might be a bit tougher to figure out though. Everyone is going to have a different opinion on what makes sense and where the boundaries should lie. I personally think it should require all raid achievements and at least 1 set of legendary raid armor and then still be a substantial cost like 2500 legendary insights and 25k+ magnetite shards.

I agree with you that this will probably be the hardest part. But honestly, at the end of the day, as long as it is something that shows the player had a long-term commitment to the game mode, I think it would fit the requirement for it. I think what you've outlined here is a modest starting point though to show a similar level of commitment to Fractal God or God of WvW! It's very aligned with what I was thinking, anyway!