Doing a rewarding Mythic+ type system for 6.0 would see subs sky rocket

[Avyiur]

I don't think Mythic+ dungeons would be a good fit for FFXIV due to how the gearing system is different however I think the old Challenge Mode dungeons from MoP/WoD would be a really neat idea considering that was only for cosmetics/mounts/bragging rights.

[Kiurai]

CM Dungeons could fit pretty well, a Mythic + style mode would probably need new and original dungeon designs to make it work well since mobs are a big factor in Mythic+'s design which isn't the way it is in XIV at least no where near the same level