What if Manamune built like this:

+

And gave this:

UNIQUE - SHOCK: Unit-targeted abilities and basic attacks on champions (on-hit) deal 2% current mana bonus physical damage. This effect only activates while you have greater than 20% maximum mana.

Just makes more sense to me that an on-hit item would be tied to, y'know, on-hit builds.

Full item looks like this:

(cost unchanged) +30% Attack Speed +250 Mana

UNIQUE – AWE: Grants bonus attack damage equal to 2% maximum mana. Refunds 15% of mana spent.

UNIQUE – MANA CHARGE: Grants a charge every 4 seconds, up to 3 charges. Each basic attack or mana expenditure consumes a charge and grants +5 mana, up to a maximum of 750 mana.

UNIQUE – SHOCK: Unit-targeted abilities and basic attacks on champions (on-hit) deal 2% current mana bonus physical damage. This effect only activates while you have greater than 20% maximum mana.

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+30% Attack Speed +1000 Mana

UNIQUE – AWE: Grants bonus attack damage equal to 2% maximum mana. Refunds 15% of mana spent.

UNIQUE – SHOCK: Unit-targeted abilities and basic attacks on champions (on-hit) consume 3% current mana to deal 6% current mana bonus physical damage. This effect only activates while you have greater than 20% maximum mana.