Hybrid Classes/Jobs

[Seraphor]

I'd actually love Onion Knight as a limited tank job.
I doubt it could slot any abilities you want from the entire game, but would rather have it's own very large set of skills, that you unlock by levelling other jobs (up to a max of level 50).

For example, getting WHM to level 30 would unlock Cure, level 40 would unlock Cure III and level 50 would unlock Benediction.
While leveling SCH to 30/40/50 would give you Bio, Bane and Sacred Soil.
AST would give you Diurnal Stance, Aspected Benefic and Aspected Helios.
Having any healer at level 30 would unlock Esuna, and any healer to level 50 would unlock Raise.
So by leveling all three healers you could effectively build a complete healer kit.

Add the casters for:
BLM: Scathe (increased potency or with a combo bonus), Freeze, Flare
SMN: Bio and Bane shared with SCH, but Summon III + Enkindle for level 50.
RDM: Corps-a-corps, Displacement, Acceleration (functions differently, grants your next 3 viable spells the Dualcast effect, so covers 6 GCDs)
Any caster: Addle for level 30, Swiftcast at level 50.

Ranged:
BRD: Windbite (can be spread by Bane), Army's Paeon, Battle Voice
MCH: Gauss Round, Hypercharge+Heat Blast, Ricochet
DNC: Windmill, Bladeshower, En Avant
Any ranged: Leg Graze and Peloton

Melee could be:
MNK: Fists of Earth, Fists of Wind, Fists of Fire
DRG: Life Surge, Jump, Dragonfire Dive
NIN: Ninjutsu+Ten, Chi, Jin
SAM: Third Eye, Merciful Eyes, Seigan (slightly different from Hassatsu: Seigan, no Kenki cost, 150 base potency, 250 under Eyes Open)
Any melee: Feint and Bloodbath

Tanks:
PLD: Spirits Within, Sentinel, Cover
WAR: Berserk, Thrill of Battle, Vengeance
DRK: Souleater, Blood Weapon, Dark Mind
GNB: Camouflage, Danger Zone, Aurora
Any tank: Reprisal and Arms Length

By default it has it's own basic tank kit including:
1-2-3 combo (Souleater can replace 3)
AoE attack (can combo into either Freeze or Flare, or Bladeshower)
Ranged attack with increased enmity (can combo into Scathe)
On-demand mitigation e.g. Sheltron/Raw Intuition/TBN/Heart of Stone
Person invuln skill (perhaps a 90% damage reduction for 8s on a 5m CD?)
Tank stance
Rampart, Provoke and Shirk

Then like Blue Mage you can slot in up to 25 of these cross-class abilities.