i would LOVE a battle back-up system.
what it would be:
characters(for example):Ean,Iya,Rye,Nicolas and Ava.
Ean,Iya and Rye have good defence but Nicolas got killed, what you'd do is use the Leader(Ean) and select Back-Up and you have 2 bars,one with your active party and one with nonactive party members.
You click Nicolas first and then select Ava and Nicolas would be dragged out of battle and be Replaced with Ava to fight on.This would give you a good way to switch out when an enemy is immune to normal attacks or magical attacks,AND it gives you a good reason to level up every character,not just 4.It would give a good excuse to buy spell scrolls for every character and it would give the makers an excuse to make the enemies slightly more difficult when you got 5 or more party members.This way there is easier to carry on in hard battles and would make it also a challenge,and you could make mini-game challenges like, Beat a challenge dungeon whit ONLY using Gavin.