what if there were 2 items with passives like these


damage above a certain amount is reduced by x percent (maybe only for physical attacks)
and
all magic damage taken is spread out over 3-5 seconds (doesnt reduce damage but protects you from being instantly burst to death)

First suggestion, that's pretty much Poppy's passive, and unless the numbers were low enough that the effect was worthless, it wouldn't be balanced at all. You'd basically be giving any champion that isn't Poppy the same level of survivability she has, which she gets that at the cost of having no AoE or farming capabilities at all.

Second suggestion, that's just screwing around really. The reason burst works at all is because it's able to take down a target quickly. If it were delayed, unless their champion was designed to deal damage over time, it would basically screw up their efforts to kill their target as fast as possible. Not to mention how it would react to DoT spells...