It took them 3 tries to find a way for challenge mode to took off and honestly they needed the help of a carrot on a stick + D3 team and that is with a background that made it easy for it to work.
FF14 feels to much stiff at the moment for mythic+ to work well, they need to sort that out before they can reliably do something like that here OR they could try and find something else, eitherway it's not something that will come out in a way it'll require time and trial and error.

Because honestly I want you to picture all the trashes in each dungeon having like 20x the health and doing moderatively more dmg, that would be like having HP sponge enemies and I don't know how much fun it'll be