We kind of had this in Stormblood. You had a tank stance with enmity, bulk and a damage penalty, as well as a separate DPS stance. Most of the DPS gain wasn't in the DPS stance itself, but rather the absence of the tank stance.
This doesn't really work, because how much damage is superior in every facet of FFXIV. Even if you are not a hardcore raider, if you can kill stuff faster, you generally have a higher chance of success and get to play more of the game in a shorter time, especially with how generous healing is. If we brought back some kind of DPS stance for tanks, unless they give us like a 75% HP penalty or something we are just going to be using it most of the time outside of dungeon trash pulls.