Simple idea that could really shake things up and give ideas for team composition, bonuses for monsters being in the same team! These would proc at the start of battle and remain for the rest of the battle, despite monster deaths and revives (so no reprocing during a revive).
If you have a two monster bonus and the 3rd monster doesn't fit with the bonus, it'll still receive the bonus, meaning bonuses are team wide, not just the monsters proc'ing the bonus.
Llum the Magical Matriarch and General Ingvar:
-Diametrical Leaders +5000 Max HP/+10% Ability Accuracy
Ouros and Igursus and Cryotan:
-Monuments of the Ages +10k Max HP
Ouros and Igursus
-Fragments of Pandalf +300 Strength/+150 Speed
Igursus and Cryotan
-Fire and Ice +10% Accuracy/+2500 Max HP
Clivia and Alpha Cliviast
-Dark Forest Pollination +15% Lifesteal/+150 Speed
There are tons of potential bonuses that could be done, and could even have an extra little monsterpedia type thing that shows all potential bonuses and maybe even a little two sentence story explaining it, gives all the monsters a little more depth.
@Fox little tag for exposure sake.