Well..there's the party bonus. They can grant a skill to all combat class that 'ignites' the party bonus stats that they're associated with. If someone activates that, the party will be informed and if someone else ignites another stat within a certain amount of time (6-12 seconds?), depending on the combo of the 2 stats, or more, they can make something come out of it. Then they'd lose the passive bonus for a few minutes, like maybe 5 minutes? Or not lose it but the cooldown will be longer, like 10 mins?

The combinations will be various, such as igniting strength and dexterity will do a single target damage on the target of the initiator, strength and vitality for an aoe attack, dexterity and vitality would stun a target, intelligence and piety would become a magical attack, piety and mind would heal the party for some amount, etc. I dont know actually remember what class grants what bonus though, and I suppose if certain combinations are really desirable, that would make people want to get a class that has stats bonus associated to that combo all the time but it would still be better if a full party can hit all 6 party bonuses and fire on all cylinders.

There's also coordination and best situations to use issues. Limit break only requires one person to initiate. A light party with 4 bonuses would need 2 pairs being aware of what to do. A full party with 6 bonuses would need 3 pairs being aware of who they're pairing with and when to use it.