While I like M+ style systems, I don't think it would suit the dungeon design in FFXIV. In WoW, each trashpull tends to have its own set of quirks that can make stacking mutators more interesting. 95% of trashpulls in FFXIV are just pure tank-and-spank.

To make it feel right, they'd need to redesign existing content to have suitable complexity, or develop new content with such a system specifically in mind imo.