To give a little more variation to the game, my suggestions
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Bonus or negative combat to teams. If attacking character in same team i.e. both red or both blue then -5% damage, opposite teams, +5% damage.
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Give each character one of 6 elemental classes. Fire, earth, air, water, time, magic put in a circle fire across from water, earth from fire etc
If attacking water with a water base -10% damage, attacking direct opposite +10%, attack elements directly next to yours -5%, attacking elements next to your opposite +5%.
In addition to that possibly opposite attacks negate effect. Frozen enemy gets hit with fire, melts ice, reverse ice attack puts out continued burning.
This will give more variety to useful characters and possibly open up more varied used instead of everyone just going to Elsa and Olaf, maybe Hades and mierda might be a better attack or defense.