Now, in the interest of not pulling a "do as I say, not as I do" I'll get the ball rolling.
WAR - Bring back BB combo (yes, it can be done, it was done with Energy Drain and Hagakure just recently). Butcher's Block now applies a DoT called "Fractured" to the target when used that lasts for 30s. While active, the DoT increases all damage done to the target by 2%, and when the DoT expires naturally the WAR also gains 20 beast gauge. This gives WAR incentive to use BB combo as a 2% raid increase and the DoT itself will both be significant, but you don't want to overlap the DoT nor do you want it to fall off for more than a second or two, so proper BB timing will be key. Storm's Eye duration baseline boosted to 40s to help compensate for this new addition. This will slow down WAR's opener a bit, but overall be a solid increase in damage, both in a raid sense and a personal sense.
DRK - Bring back Power Slash combo. It grants a 6% haste buff to DRK for 30s while Darkside now only grants 5% damage (net increase of ~1% overall damage for the DRK). If used under the effect of Dark Arts, Power Slash now also applies a debuff called "Shredded Soul" that increases damage done to the target by 2% for 60s. Shredded Soul also causes all damage done to the target via weaponskills/spells to replenish 8 MP to the DRK who applied the debuff and 2% of the damage dealt as life to the person who did the weaponskill/spell. Assuming a 2.0 GCD, this would be 240 MP over the course of the debuff, or approximately 20 potency. At 2.4 GCD you're looking at 200 MP over 60s, or approximately 32 potency. This means over the course of the 60s, assuming 2 tanks and 4 dps attacking a boss non-stop and 2 healers occasionally attacking between heals, you'll see maybe 600-800 MP gained, or about 250-280 potency extra. 2% extra healing may not sound like much but over the course of a fight it would add up to some significant healing, saving healers at least a few extra GCD's over the course of a boss.
PLD - Bring back Rage of Halone baby! As with PS and BB combos, this will put a 2% increased damage taken debuff on the target, as well as the "Blinding Light" debuff for 30s. Blinding Light makes it so that anyone within 5y of the PLD takes 1% less damage from all sources, while the PLD takes .5% additional damage for each person who benefits from the Blinding Light effect. A full raid stack would have everyone taking 1% less overall damage while the PLD takes 3.5% more damage. A typical "solo MT in front" situation would see PLD damage taken unchanged, while pairing up next to the OT for a mechanic would cause a negligible .5% incoming damage increase. 1% damage may not seem like much, but we've all known that dps who survived with single digit life before; every bit of mitigation helps, but it's not so much mitigation (nor is it easy mitigation to manage outside of general raid stack stuff) that it gives PLD too much of an edge.
GNB - Add a second cartridge combo. Call it "Trigger Blast" or something, I dunno. Potency-wise it would be about 80% of what Renzokuken does (not counting Cont.) but it would instead refund the cartridge used upon successful execution and then apply the "Mauled" debuff for 30s. As above, Mauled would increase damage taken to the target by 2% while also reducing damage dealt by the target to the GNB by 5%. GNB damage as-is feels pretty good; the addition of a combo that's 80% as strong as Renzokuken (and also refunds the cartridge used) is already a stout little dps bump, so the Mauled debuff plays into that by addressing GNB's key weakness of being the squishiest of all the tanks. It's chief benefit is while MTing of course, but it's also useful for taking the edge off of raid-wides and shared busters.
All of these 2% damage increase debuffs would stack with one another, but never more than two separate instances at a time. This means you could run any combination of two tanks, but only up to two tanks and reap the full benefit from the debuffs. Three tanks (or more in the case of big stuff like hunts or FATE's) would still get the personal benefit from their debuffs, but the damage increase would be capped at 4% regardless of how many debuffs were placed on the mob.