Combo abilities

Options
Simple Short Term Version
Take lesser used characters and refresh them by taking two existing abilities, and combining their requirements (X amount of color Y for ability A, and X amount of color Z for ability B) to have a greater overall effect than the individual abilities themselves. Eg; Modern Cap's yellow combines with OBW's blue to create shield tiles and heal as well as stun opponents for 2 turns.

More complicated long term Version
Combo abilities do not leverage existing abilities and are entirely new abilities that require single or multiple color types. Combos can exist between 2 or 3 characters on the same team.

Think Marvel vs. Street Fighter combos in MPQ.