At this point, the items shouldn't even give actual damage stats. "Makes you do 20% more damage" and balance champs around their base damage. Some items are made for specific classes because otherwise, the balance would be completely thrown out the window.

Remember, we had an AD version of Mejai that was removed for a reason. If Zhonyas gave AD, every Renekton and Riven would build it. Have you considered the nightmare Ryze would be if Manamune gave AP instead of AD? Ever thought about AP assassins wave clear with AP hydra?

Items are not interchangeable for an obvious reason. Only a handful of them would actually be beneficial to be reworked as something like that, and even then, it could become a problem in some unpredictable way.