Here are some ways they could keep the ATB in the game, but more fun and realistic.
-Enemies show damage. The more damage an enemy has, the more damaged it appears.
-Able to target specific body parts. Head hits can cause enemies to miss more often, leg hits can cause enemies to dodge less often, arm hits can cause an enemy to do less damage, body hits can cause an enemy to buckle over and lose attack turns.The same effects can happen to your characters.
-Put a greater emphasis on character placement. Not just in 3 rows, but in a 3D space.
-Give different potiions a toxicity cooldown to avoid over-reliance on it.
-Have super versions of each enemy that is a much bigger challenge to kill, regardless of your level. These enemies would scale to your level to some degree and can randomly occur in place of the normal versions.
-Use a mechanic similar to FFX-2 so that chained attacks can cause extra damage, and the same can occur to your characters.
-Different spells/attacks should be leveled up the more you use them for different traits.
-Introduce battles that require more than 3 characters. Not every battle should let you use the full roster, but key fights like the optional bosses and Sephiroth/Jenova should allow you to use your whole roster. This would allow them to make these boss encounters much grander in scale and difficulty.