So, no items. I.e. nothing to equip. The change comes from selecting the monsters that will be fighting. Each on having (I assume) a very limited and very focused nature.

Here are a few loose thoughts.

1) Perhaps the ability to change a character mid fight. Perhaps with some stipulation, like their health must be maxed, and that monster uses their action for the round.

2) You didn't express much about the options they do have, So here are a few considerations.

- use a round to power up any action, I.e. Wind up for a punch, so the punch does 3X damage.

- Take a Hit for/from X. Some monsters might be really strong at taking a beating. Perhaps you could have them guard a character, so an enemy has to hit them first. Or perhaps they could defend from a particular enemy, no matter who it is going after.

3) Perhaps you could have it be 1 on 1, but with X number of characters that could switch out as their action. And first to no characters left, or first loss, loses.

4) if it is multiplayer, FF timer would be great, so you don't have to wait on X player to make their attack decision.

5) Do the monsters Get XP from being in a fight? Perhaps team up a weaker monster with a pair of veterans, and take on something easier, so it gains XP.

Another consideration though, is the differences between Desktop and Mobile. I.e. Mobile players are typically looking for something they can start in a few seconds, and drop at any instance. I.e. waiting in line, board for a moment, just wasting time. When compared with desktop/console, the play style is usually more engaging/in depth. Not making any suggestions, but just keep the modes of game play in mind when targeting systems.

In a team battle there is an obvious way to increase complexity: the whole team fights together, without swapping monsters (which tends to be a fairly boring move) and each attack targets a specific enemy monster, retaining the same abstract or trivial positioning of Pokemon games and a similar display (the whole teams, everybody's status, and turn order).

I see three main possibilities for basic turn structure:

• Actions taking a variable time, every monster acts when ready: the same team, or even the same monster, can have consecutive turns, but only as a temporary result of choosing very quick and presumably weak actions. Obviously appropriate if there is a wide range of actions of different durations (brief, normal or long) that shouldn't all have the same cost of "one turn".
• Fixed order (team 1 monster 1, team 2 monster 2, etc.), suitable for predicting what will happen many turns in advance and therefore appropriate if move effects have little or no randomness.
• Flexible order (alternating teams but letting the player choose which monster acts, freely or among those who haven't moved this turn), suitable for optimizing move choice beyond a single monster and appropriate if move usefulness varies a lot according to the situation.

Concentration of attacks against a certain monster could be limited if necessary with simple rules (cannot attack the same opponent as the preceding teammate in turn order because they are still engaged, cannot attack the enemy with the least hit points or any enemy that is stunned, blinded etc. because it's not nice, and so on).

I would expect a significant importance for buffing and healing moves affecting teammates, for delaying and disturbing opponents, and for combinations of attacks from multiple monsters (which can be contrasted with the narrower tactical options of single monster fights).