Do you plan to have a certain way to force the player to play with more than almost always the same 4 characters the whole game?
FF4 FORCES you to change your party by removing and adding characters through plot. VP doesn't force you, rather it strongly encourages you to change up who you use.
For example, say characters must be used in combat to level up, but when not in combat they meditate or analyze the fights they watch, and can practice...so they'd receive a bonus to their XP gain for each fight they do not participate in, and that bonus would decay with each fight they do participate in. Thus to more quickly level up a character they'd have to sit out, whereas if you kept them in constantly they'd still level up but much more slowly.
Or it could be things like they level up by being in a fight, but levels don't give you extra Action gauges, so to get those they'd have to meditate by watching battles.
Etc.