Here's an Idea: Give each Class a general action called "Skill Chain" and "Skill Burst" that can be used off the GCD with a 30second cooldown.
Skill Chains can be used to Support the party via damage buffs / healing buffs / defense buffs while Skill Bursts will damage your target / nearby
targets.

Skill chain
If Player 1 activates the "Skill Chain" before using a skill (Skill chain>> Heavy thrust) It places a buff on the party called "Chain 1" for 7s.
If Player 2 activates his/her Skill chain and uses the proper skill (Skill chain>> Twin Snakes) before the buff fades, the buff becomes "Chain 2" and increases party damage by 10% for 10s
If player 3 activates his/her Skill chain and uses the right skill (Skill chain>> Medica II) before the buff fades, the buff becomes "Chain 3" and increases party damage by 20% for 10s

Skill Burst
If player 1 activates the "Skill Brust" before using a skill (Skill burst>> Fire III) the monster receives a de-buff called "Burst 1" for 7s
If player 2 activates his/her Skill Burst and uses the proper skill (Skill burst>> Rockbreaker) the monster will take damage equal to that players potency of 200 and receive a de-buff called "Burst 2" for 7s
If Player 3 activates his/her Skill Burst and uses the proper skill (Skill burst>> Holy) the monster will take damage equal to that players potency 350 and the burst chain will end.

I MISS SKILL CHAINS!!!!!
Just an idea.