I could definitely see them cutting it, in effort to make the holiday 2015 deadline, tbh.
Designing entire battle systems for each member seems like a shitload of work (an ability that made me most hyped for this game). I could see them saying "fuck it" and default to this new whatever-it-is.
They already seem to have a buttload of movesets for the characters, so I'm not sure if it would be all that much work just to make all those moves playable, especially since they pretty much have to think about the handling & balancing of all the different kinds of weapons because Noctis can use them. I mean, they already have all kinds of movement animations for the characters, would it be all that much work to give the player control over them? And they already have plenty of different kinds of attacks.
They could make Noctis the main star but give essential support roles for the other characters. I never expected the other characters to have the versatility of Noctis, who Nomura was always talking about as a jack of all trades & master of none who could use all the weapons and zero-shift around while others would be tied. More in the line of the others just being a little more efficient in using their speciality weapons for added benefits & special skills. So while Noctis CAN swing huge-ass swords, maybe Gladiolus would be the only one to have enough strength behind his swings to shatter some defensive layer of some bigger enemies. And while Noctis could maybe use guns, maybe Prompto would be accurate enough to damage airborne enemies in a way that makes them fall down so that the more ground-tied characters could attack them. It shouldn't be overly taxing to give Prompto a few different cool gun-shooting poses for different types of guns he could alter between and maybe make him more of a Vanquish like character who stays behind cover and has to keep himself away from attacking melee-enemies with a little more efficient dodge-rolling/other evasive moves. ANd while I mention Vanquish, I never expected FFXV to turn into Vanquish level 3rd person shooter whenever you switched to Prompto, just offer a slightly more basic (lock-on including) 3rd person shooter to bring a little bit of variety to the combat and Prompto having his own useful skills that you would WANT to switch over to him when it feels wise.
Basically, give all characters some basic 3-5 hit combo (they already have the animations, just make it so they are behind button presses), maybe make each of them good at different branches of magic (Ignis would be an efficient Fire-caster with maybe the strongest fire spells that even Noctis can't use, Prompto could be Lightning, Gladiolus some Earth-based spells and Cor a master of Ice), a few exclusive skills that are suitable for their weapons/skills/quirks (i.e. Gladiolus could get rid of smaller enemies efficiently with the supplex move we see in the 2013 trailer and we know that Prompto had the ability to see "weak spots" in enemies & attack them in early builts of the game's combat system etc.)
I wouldn't be opposed (and actually thought the game would be like it) of the game having Noctis be the go-to character you use the most to move around & all that and then you just switch to other characters occasionally to strip away the enemy's shield or do something else Noctis can't do to maybe put an enemy into a weakened state and then switch back to Noctis to deal damage. Didn't really think about the game as being the kind where you'd want to play it through with only using one character, but more of a team-combat system where Noctis is the leader and the others have some support actions that you also have control over.
I'm just super skeptical as it is. Tabata, had zero reservations about completely upending Nomura's vision for the battle system, which still seemed in place back at the E3 2013 trailer (we even had a breakdown analysis done by gaffers. It was super promising)
Tabata mentioning "only controlling Noctis" even if he's just referring to the demo, make me uneasy. Why would such a crucial feature not make it into the polished demo? Either it's coming in really hot, or they nixed it =\
Demos often limit the degree of control, whether it's by limiting the moveset or the amount of characters you can control. Maybe Noctis gameplay is the furthest along in a way that they can give gamers a look at a slightly more powerful late-game Noctis whereas the others are more WIP so they'd rather just let the AI handle them for the time being.