How about the combat system is very similar to pokemon with 2v2 battles. IE you select an action for your first character and then your second character. Then the turn happens where your first character attacks, the enemy's first character attacks, your second character attacks, the enemy's second character attacks, and then the turn ends.Yeah that sounds good, except the order pokemon go in is determined by speed, its not just systematic. It would be cool to have a speed stat which does the same thing.
That's exactly like pokemon.
Unlike pokemon though, the rest of the party members aren't safely stored in pokeballs. Instead they are a part of the fight too, essentially acting like a third character.
You select an action for your first character. Depending on the attack it can be directed at either of the enemy's front 2, both of the enemy's front 2, or characters on the enemy's sidelines. Then repeat and select an action for your second character. You then select a third general action for all your side characters to perform. This is a simple thing like "defend," "attack," or "scatter." You don't decide who they attack, instead it's spread out over all enemy characters.
Combat is essentially 3v3, yet the third fighter is a conglomerate of all your remaining party members. That way every party member is always assisting in battle, and increasing party size doesn't necessarily increase fight duration.
We'd just have to think of the different abilities and how the damage will be determined.