Just thinking... I'd love to craft "cursed" onto an item. An invisible attribute that allows you to choose another say 4 slots that only apply to the item if it transfers ownership. They'd be invisible and have an activation. or passive feature. Imagine looting the one ring, equip it and get 'cursed' and 'passive wraith transformation'. And now you have to decide, it gives +++atack, ++health, ++def etc but you have to gamble with it... you already know that after 'n' turns it'll turn units into wraiths, so can't be equiped forever safely. You could find out the other features of it through a few battles. Say you find out 1 of the cursed attributes after 1, 2, 4 then 8 battles. Really you'd have a choice: keep it for later, destroy the item (and not gain any mana from it), use lots of magic to identify it... then use magic to cleanse it, or use real battle experience or time to identify it for free (it may be a very mild curse) but risk one of the attributes being activated (by healing the unit for example). Imagine a 'healing disabled' and 'poison' curse kick in only when you try to heal your One ring wearing and otherwise amazingly powerful Gandalf mid war.
What about some of these cursed items being given as gifts. Give an ally a ring that passively drains mana from that empire to you. They would obviously destroy the item unless it was powerful enough to be worth it... or pass it on, or lose it, or cleanse it. Perhaps they could use it against you or perhaps it's beyond their ability to forge. Maybe you expect them to cleanse it and plan for that (use is to occupy them magically) or know they can't because the curse is too strong. I hate 'can't unequip' curses... or 'drains health' I'd much rather have them make you really think. Maybe a race could choose as a perk to gain a default curse, it would make those really nasty cursed items that much harder to spot against a sea of -1 attack curses.
Edit: I know this isn't really about heroes as such and it's more revisiting items. But still, knowing that some of the artifact and equipment attributes could be quite strange and interesting is important to making it fun. I'd like to be able to find and make sets as well. Say you can enhance an attribute up to x, if you add anything more than that it'll only be activated when another element in the set is present. So a pair, ring and helmet of [stone]... would have +x defence alone, and +y when together. or even have an extra ability +y per additional item with "set of [stone]" attribute. Maybe have stronger item attributes require another 1,2,3... items to activate. Something to make item sets and artifacts different. Maybe if artifacts could have the powerful spells in and all those spells could only be reached by having a set of 5 otherwise. So you could have the one ring as an artifact and a set of invisibility requiring 5 [air]/[darkness]/[evil] items... not sure how to work it, what sets groups there would be and which spells would fit in each set.