Also, why not we have scrolls that can be dropped by mini-bosses, bosses, elite and epic bosses that can be used on an item to increase the stats of the item, buffing it? That'll be quite cool and makes things personalised.
Since WC3 items are static, if you were to make 'upgrades' for items, you have to make a new item for every upgrade, which will surely bloat the maps even more.