Ok so we get five throwaway dungeons and three trials per base expansion. They did hard mode dungeons back in the day which reused environments, they weren't actually any harder however, just more boring one and done content, at max you would soullessly grind them because of perverse incentives.
Dungeons are not good at being evergreen content, but criterion dungeons potentially are! With the right reward structure yada yada.
Let's say they design Siege of Doma in Stormblood with Variant dungeons in mind.
You do the normal Doma dungeon during the MSQ with all the action and the spectacle, normal route, other paths are blocked and irrelevant.
However at maxlevel for patchcontent they can reuse the same environment and alter and expand it a bit - for example the palace is now sunken after the events of MSQ - for a variant dungeon!
So you go back there to secure some family heirlooms and items of cultural importance or whatever, it's low stakes and chill and focuses on the city and Doman history. You can redo the prior path but instead some gates won't be blocked, you can just sidestep the palace for example and explore more of the exterior, take a hidden underwater passage, monsters have taken root, one boss could be a crazy lone ninja fighter, one a half broken garlean piece of tech, one some sort of kami from a trinket like Susano or Tsukuyomi.
And then of course you design criterion and criterion savage on top of that.
Now you don't wanna creatively hamstring yourself by not being able to design unique variant dungeons anymore or having to return to a dungeon. But in general this I think would be a good way to save some workload. To be honest I consider one and done dungeons a bit of a waste of resources.
I know assets are reused anyway but being able to reuse an entire environment (you can change the atmosphere a lot by just altering the music, skybox and ambient lighting btw!) could be super beneficial.
Generally speaking almost everything in the game is better than dungeons. Trials are better, deep dungeons are better, variant dungeons are better, raids are better. Dungeons suck. And it doesn't matter if you call them hard when all they effectively are is more dungeons. However reusing assets is good if you can save work. If they can be reused for more interesting content, I would adore. Goes for the maxlevel dungeons as well btw.