Why can't they just upgrade FFX-2's ATB system? That game had the most "realistic" ATB system of all the games. Not to mention it's by far the most fun to battle with.
		
		
	 
This, this, this. 
Makes the most sense in any regard: 
- 
Nomura - rightfully so - doesn't want the party to stand around on the same spot, jumping back and forth, and the FFX-2 system would solve that problem
- it's not too technically ambitious (FFXIII basically built on FFX-2, too, with automatically moving party members), especially compared to something complex like FFXV's fully action-based system (huge amount of work going into collision-detection, animation-systems, AI/path-finding, etc.)
- also makes it easier for scripted, flashy stuff happening in-battle, like QTE-battles in FFXIII-2, than with an action-based battle system
- if it's menu-driven and you have control over the whole party, it still would feel oldschool enough for the conservatives who are afraid of change
- combined with seamless battles ala. FFXII it would remove the annoyance of random encounters (some here really want it back, seriously!?) and make it feel less outdated for the mainstream audience - of course, the level-design would have to accommodate with larger areas and some maps would be better off without monsters roaming (f.e. the Shinra Mansion should just be an atmospheric, creepy place straight out of a horror game, without battles (those swinging-scythe dudes would look hilariously out of place, haha))
- no gambits/automations like in FFXII/FFXIII, though, as that will make it 
feel more shallow and uninvolving, even if otherwise the game, like most FFs, could be mostly beaten with button-mashing, too, anyway (having to chose the commands yourself makes you think you're doing something strategic, even if it's the same simple 
rote memorization patterns all the time, e.g. "use fire against blue enemy" or "heal if HP low")