That's not a terrible idea, but it uses D&D/Pathfinder logic over Final Fantasy logic.
In other games Necromancy is used for healing and raising the dead. In Final Fantasy the "Necromancy" spells are all treated as evil and anti-life. The undead take damage from healing magic and are outright killed from raise dead spells.
Necromancy in Final Fantasy would wholly be DPS or possibly Tank. As in draining enemy health to heal themselves.
FYI - The Final Fantasy Necromancy spells are:
Demi - Reduces enemy HP by a percentage (probably too powerful for an MMO)
Drain - Deals damage and self-help for the same amount.
Doom - Percentage chance to kills target instantly - Too powerful for an mmorpg.
Sever - Appeared as various incarnations, this turns a target into a zombie that attacks mindlessly and inflicts the zombie condition that grants the same weaknesses and advantages as undead.
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I'm a lorephile - So I want to see new things added that are derived from the existant Final Fantasy lore whenever possible.
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Some people mentioned "Advanced Classes/Jobs" and I'm down with that as an idea. There was precedent for this in the original Final Fantasy. After finding the Rat's Tail artifact for the dragon Bahamut the party was empowered to new classes.
Warrior -> Knight (Paladin)
Monk -> Master
Thief -> Ninja
White/Black/Red Mage -> White/Black/Red Wizard
As these are all upgrades to base classes, I'd like to see something similar for the next expansion, but focused on the base jobs.
Warrior, Paladin, Dragoon, Monk, Ninja, Bard, Black Mage, White Mage, Summoner, Scholar
So, how I would do it, would be something like:
(All require level 80 base class and start at level 50)
Warrior -> Weapon Master
Paladin -> Holy Knight
Monk -> Master
Ninja -> Shinobi
Bard -> Spellsinger
Black/White Mage -> Black/White Wizard
Summoner -> Incarnate (basically channels summons through themselves)
Scholar -> Philosopher
(I had to invent names for some of these since they had no FF comparison.)
Each would make the base job into a hybrid. Basically base class, so the abilities of the base job to 50, then new role powers from 50-level cap
Weapon Master: Tank/Melee DPS
Holy Knight: Tank/Defensive Buffer
Master: Meele DPS/Tank
Shinobi: Ranged DPS/Debuffer
Spellsinger: Ranged DPS/Healer
White Wizard: Healer/Ranged DPS
Black Wizard: Ranged DPS/Offensive Buffer
Incarnate: Ranged DPS/Defensive Buffer
Philosopher: Healer/Offensive Buffer
Basically the "secondary role" works in a passive manner - Similar to how jobs like Dancer are Ranged DPS/Offensive Buffer.
So Weaponmaster would be not quite as tanky as a tank (but still tanky) but with more damage.
Holy Knight would be not quite as Tanky, but have spells that up their, and their party's defenses.
The Master would gain some tanky abilities. Etc etc