The biggest reason for me wanting ATB/Turn Based combat back is because it allows you to control the entire party and strategize based on party composition/available skills. Action combat is always just controlling one character while the others run AI. If the AI is shit, then it makes it feel like you're gimping the rest of the characters, if it's too good, the game plays itself. Very few companies/battle systems hit the sweet spot. I think FFXII's Gambit system is probably the best in this regard, if you're only controlling a single character in combat. At least then you are the AI programmer.
But yea, a big budget ATB (with new bells and whistles) Final Fantasy is the dream for me. As others have said though, the story, presentation, and characters matter a lot in the big picture too. Like I don't think Turn Based FFXV would have been any better of a game than the one we got.