I would like to see a real supporter in the game. The way I would do it is:
All his skills do have about 20% less potency than any other DD´s.
His skills support different classes. Any skilleffect lasts 18 seconds. That way he has to choose if he supports up to two classes close to the maximum or if he splits his attention.
He supports tank classes by:
Combohit one - no benefit
Combohit two - slashing resistance of the target is reduced by 5%
combohit three - aggromultiplier of any skill that hits the target is enhanced by +1
He supports Casters by:
5 second casttime-skill: Area of effect: Any enemy in the area takes 15% more damage by magic damage.
(I know Blm gains more. Wait for it.)
He supports Dotters by:
Instantcast singletarget. Any dot active on the target got a +15% chance to hit critical with a 2.0 multipier.
He supports Dragoons/bards/monks by:
combohit 1 no benefit
combohit 2 reduces piercing resictance by 5%
combohit 3 reduces piercing resistance by an additional 5%
Bonus effect if hit from the side: melee attacks done from the side do 5% more damage
He supports monks and dragoons by:
combohit 1 no benefit
combohit 2 reduces blunt resistance by 5%
combohit 3 reduces blunt resistance by an additional 5%
Bonus effect if hit from the side: melee attacks with any positional modifier do 5% more damage
He supports Healers by:
2.5 second cast targeted: Target recieves 1% of the damage done as hp (maybe 2% but it should be weaker than a crossclassed bloodbath)
Instant targeted: The next heal that hits the target gains 50% more potency. Pacifies self for 7 seconds.
He supports bards by:
toggle ability 2 sec cast: pays half the cost of any bard song active and takes half the malus the bard/s may suffer.
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Taken that combohit 1 would be one skill for all that would sum up in 12 skills opening room for some debuffing skills and ofc a silence.
For crossclass I would give him arcanist and pugilist to enhance his supportskills even more with Virus Manta and e4e.
His damage should be unassinged so he can´t profit from his own buffs.