At the moment, magic items seem a little sparse and all over the place.
It'd be nice to vastly increase the options of magic items (elemental enhancements for weapons, etc. arrows of dispelling, etc.)
However, this is geared at certain high cost end-game items that should be world-shattering that would be unlockable through magic research or questing.
These items would require some sacrifice on the part of the sovereign, such as essence (assuming that's around with the new magic system planned, if not hitpoints) and special materials such as elementium.
The items would be like wonder buildings, one can be built at a time and be at play in the world.
Some items:
Ring of Domination- Gives your sovereign diplomatic bonuses with known nations using a subtle enchantment. Can be cast on hostile heroes or monsters (non-sovereign) to subvert them to your side. The turn cooldown for this charm ability is tied to the level of the target divided by 4. (So a level 20 target would take 5 turns to recharge for its next use) Overall there's a choice between rapidly subverting low skilled opponents or slowly taking the highly skilled ones.
Throne of Doom- This device is built at a settlement and is a fusion of magic and the darkest technologies of the Titans. It fires an orb of energy that will utterly annihilate anything within a 4 tile zone in a mushroom-like conflagration of magical fire. The device can be fired at anything with line of sight. (seeing spell can be used to target for it) It has a 20 turn cooldown that can be hastened by drawing essence from a channeler (1 essence for a turn). (as strategic spells *should* have, using this should have huge diplomatic repercussions)
The Dark Soul- This when constructed is an orb held by a hero that can consume an organic unit and convert it into the undead. Undead units no longer have upkeep, and have immunities to certain spells and abilities. (things such as fear, morale loss, etc.)
Anyway, ideas like this. Big things that can change current game mechanics or play with them.