I've been playing a lot of Dragon Age II and not a lot of FFXIV as of late. Not because I dislike FFXIV or anything, just letting you all know where this idea came from.
One thing I really like in Dragon Age II is the combat. DAII really makes it a point to have the different classes work together. I think that FFXIV could borrow from the DAII Cross Class Combo system for some pretty cool results if combined with elements of the current system.
How it works: Each DOM/DOW class (not weapon skill/ability) will have a status that it can inflict on an enemy These statuses each have a small debuff attached to them - not the real benefit.
Certain other class moves (weapon skills/abilities) will take advantage of these weakened states for large increases in damage, consuming the status and debuff. Only one class based status effect may be active at one time. They may be consumed and replaced by a single ability.
All class based statuses are inflicted on a % chance basis. Base values apply for moves used to inflict the status inherent to the class (Shadowstich used by ARC). -10% when a different class' move is used (Spinstroke used by ARC).
Example:
ARC - Status: Punctured (5% Slashing damage increase)
Raging Strike: When used now adds - a +20% chance to inflict "Punctured" - to next move to consume the Raging Strike effect
Hawk Eye: When used now adds a +10% chance to inflict "Punctured" to next move to consume the Hawk Eye effect
Puncture (WS): When used has a 60% chance to inflict "Punctured" on all targets hit
Passive: 5% chance to inflict "Punctured" with every attack
So an Archer using Raging Strike w/Puncture would have an 80%-85% chance to inflict the "Punctured" status. Using Raging Strike (+20%) and Hawk Eye (+10%) with Spinstroke (-10% non-ARC skill) would only have a 20% chance to inflict the "Punctured" status. If inflicted, the target(s) will suffer 5% more Slashing damage for the effect duration.
The cool part comes in when certain skills and abilities from other classes exploit these statuses to increase the damage of their own attacks. The key is that no class can inflict the status effect needed to significantly augment their own skills' damage. They have to rely on another class.
Example:
LNC:
Trammel (WS):
+75% damage if target(s) "Battered" (MAR) or "Doomed" (THM)
+50% damage if target(s) "Punctured"or "Rocked" (PUG)
In a party with an ARC, the LNC would be able to capitalize on the "Punctured" status the ARC inflicts if they react with the WS quickly enough. Taken a step further, if used in a Battle Regimen, the ARC and LNC could combine for devastating damage.
ARC buffs with RS and HE -> Battle Regimen -> Puncture ("Punctured" inflicted at 80% chance) -> Trammel (receives BR damage bonus and +50% from consuming "Punctured" effect)
Longer chains, some planning and a bit of luck could lead to really crazy combinations and serious damage. It would also bring about some type of class uniqueness and synergy. The key to balancing these would be making it so that it takes skill to set up long multi-class chains but only a moderate skill level to combo off of a random proc.
Thoughts?