u/Koshindan* came up with an amazing idea for implementing unique variants of skills from items:

Give some items special Oskills which replace skills in your skill tree while equipped, with different synergies than the original skill!

I like this idea because it can lead to some fun combinations that haven’t been seen before. I made a few example ideas for items that could use this mechanic.

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Unique Stalagmite (Staff) – Northman’s Fury

- Replace Whirlwind with Cyclone (Barbarian only)

Cyclone: Uses Whirlwind animation. Converts 50% of physical damage to cold damage. Lower physical damage scaling than Whirlwind. While using this skill, you are surrounded by a Hurricane which does flat cold damage. Counts as a Cold skill.

Synergies:

• Increase Speed: % cold damage
• Natural resistance: % cold damage

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Sorceress Orb Runeword – Terror

- Chance to cast Terror on cast.

- Replace Inferno with Lightning Breath (Sorceress only)

Lightning Breath: Uses Diablo Lightning Breath animation. Deals lightning damage. Counts as a Lightning skill (for items like Thundergod's Vigor).

Synergies:

• Combustion: % lightning damage
• Fire Wall: % lightning damage

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2-handed Weapon Runeword - Fervor

- Replace Charge with Battering Ram (Paladin Only)

Battering Ram: Uses the Charge animation. Rams through enemies, knocking them to the side and damaging them. Attacks with a Shockwave at the end of the charge. Better damage scaling than Charge. Requires a 2-handed weapon.

Synergies:

• Vigor: % shockwave damage

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Unique Overseer Skull (Necro Offhand) – Skull of the Conqueror

- Replace Bone Prison with Skeleton Prison (Necromancer only)

- Replace Bone Wall with Archer Wall (Necromancer only)

Skeleton Prison: Summon a ring of Skeleton Warriors which attack enemies inside the ring (no splash damage).

Synergies:

• Archer Wall: % physical damage
• Bone Armor: % physical damage
• Skeleton Mastery: health for skeletons

Archer Wall: Summon a wall of Skeleton Archers which shoot directly forward.

Synergies:

• Skeleton Prison: % physical damage
• Bone Armor: % physical damage
• Skeleton Mastery: % chance to pierce

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I'll include some details about the mechanic in a comment so I don't make this post too long

Feel free to comment any feedback or ideas of your own.