I run www.runic-paradise.com, a Minecraft survival server. I have thus far coded our custom plugin myself, and it's probably a mess to others... this isn't the latest version but it is a recent one here on github:

https://github.com/runelynx/RunicParadiseOnline

I am now adding a "Faiths" system where players can level up particular schools/faiths and over time accrue certain abilities - mostly passive but some active. Things like :

>On player receiving damage from a Monster entity, 2% chance to set Monster entities within 8x8x8 range on fire
>On player damaging Monster entity, 5% chance to Slow nearby Monster entitites
...etc

I can handle these pretty easily with my knowledge. But there are other spells where they get more advanced than I'd like to get into... an example being maybe growing crops in a certain radius around the player. My forums are down but there's a small wish list of specialized spells there from the players. I would love for a "spirit weapon" type spell, where a "floating sword" (or invis zombie w/ a sword) spawns and defends the player.

I'm sure the complexity is what determines your assistance... but does this sound like something you'd be interested in? My server does NOT bring in major cash but the pricing on your post looks acceptable to me. In general I might have 6-12 spells to add on your side (I'll still be working on the simpler ones).



SAMPLES:

Time: Idea #1, Rewind: (the restore some health upon death and not die was my idea, so that for this but I'll go into more detail)
 Upon taking fatal damage, instead of dying, would 'take you back in time' (so to speak) restoring X hearts and grant immunity for
a very short time frame to ensure the passive isn't wasted (based on rank as well, nothing more than 1 second though)

Fate: Idea #1, InevitableDemise: If your targets takes damage that would reduce them at or below 10% max health, they will be executed
for attempting to 'avoid' their inevitable demise. Has a X% chance to proc and has a X minute cooldown. (on bosses such as Wither bosses
 and ender dragon, the max health percentage is instead lowered to 5% and the percentage chance is halved)

Nether: Idea #1, NetherBorn (or Abyss Born your choice): Have X% increased damage, and take X% reduced damage while in the nether.
 Idea #2, Vampirism: Grant X hearts every X kills. (Passive animals like cows, would only grant .5 towards the X kills)

Aether: Idea #1, Angelic: Have X% chance to take X% reduced fall damage, Upon sprinting grant a 20% speed buff for X seconds (X minute Cooldown)
Idea #2, Frenzy: Grants X% speed buff that lasts for X seconds every X kills

Nature: Idea #1, Cultivator: Crops and saplings that YOU plant grow X% faster. Food harvested that YOU planted have X% chance to double drop,
 Food eaten that YOU harvested restores X extra food bar. (You basically have to do everything for any of this to actual benefit)

Tech: Idea #1, ElectroShockTherapy: Have a X% chance to 'shock' your target, and deal extra damage, also applies a X% slow and a X% damage
 debuff to afflicted target for X seconds. (would work on bows and swords) Idea #2, TechnologicallyEnhancedArmor: Have a X% chance to block X hearts
 amount of damage from all attacks

Time Abilities: Hunger Bar goes down more slowly, ALL potion effects last 1.5× longer.
Time Spells: "Flash". Swiftness 1 and haste 3 for 10 seconds.

Fate Ability: johnnywaffles' idea is really good. But make it apply to the player, too, for balance. Lose 9 hearts of health, and instadie ;)
Fate Spell: "Zeus' Anger" Rain lightning on the unfortunate soul that decided to mess with you.

Aether Abilities: Hunger bar only needs to be 80% full to regain hearts. Only needs to be level 25 to get level 30 enchantments from the enchantment table.
Aether Spells: Arrows deal 25% more damage (cos cupids lel), swiftness 2 for 10 seconds

Nether Abilities: Same idea as johnny's, but with the inverse for the overworld.
Nether Spells: Set all hostile mobs in a 5 metre radius on fire, fire resistance 3.

Nature Abilities: All sorts of meat give half the hunger bars they usually give, and all sorts of non-meat food give triple the hunger bars when waten. Planting a sapling regenerates a heart. Wooden weapons deal 3× more damage.
Nature Spells: instantly harvest a row of crops.

Tech Abilities: Repairing items on anvils cost 10% less experience. Iron Armour gives 50% more protection, and 10% chance of dealing damage back to enemy, but other armours give 25% less protection. Mining ores give exp.
Tech Spells: *Using a bow* Rapidly shoots arrows for 5 seconds.