Honestly? Aside from needing a target to use them, nothing is wrong with cone AOEs by themselves. It only really becomes a problem when they're linked to circle AOEs, and only for DPS Jobs because the meaningful range of a circle AOE is lower the further you are from the center of a group of enemies, resulting in a playstyle of hotfooting back and forth from the edge of a pack of enemies to the center of them.

Tanks (actually just WAR) didn't really have to worry about that, because enemies should be clumping together in the space in front of you. If they're somehow attacking you from behind, you've screwed up. But in that case, I'd say Tanks have the opposite problem: Circle AOEs are outright worse than Cone AOEs in all ways except "needing a target to us them". Which could easily be solved by removing the need for a target.

Also, if we wanted to introduce even a smidgen of variety in how Tanks play, they could make each Tank's AOE skills act differently. WAR's could be cones, DRK's could be lines (like Dragoon's or Flood of Shadow), PLD's could be full circles, and GNB's could be half circles. Their animations would have to be adjusted to match the new hitboxes and their ranges would have to be rebalanced, but I think it'd be a neat change.