I would still love to get some version of Bloodrazor back
The item's biggest failing was that it got pegged as an ADC item despite having a pretty wonky selection of stats (armor, IAS, AD, and OnHit damage). So I would like to suggest something a little more specific. A magic damage version of Death's Dance (sort of).

Grants attack speed, health, MPen, ability power, and-! Unique Passive: Your basic attacks deal 4% of the target's Max HP in magic damage OnHit. Unique Passive: You heal for 15% of the magic damage you deal. One third effectiveness on AoE abilities.
The main problem with AP Marksmen itemization is that, honestly, there are only two items to support it (
). That's part of the reason why Kayle becomes broken as fuck every time a new item is released that she can use. Because she's 'balanced' around having shitty itemization choices. With robust AP/OnHit/Attack speed options to choose from, Riot might finally be able to make her manageable.
Along with this new item, I would like to suggest allowing magic damage OnHit effects (not true damage, sorry Vayne) to deal damage to Towers. That would allow a Mage, if the player were so inclined, to run an AP Marksmen build and take the bottom lane position without their team losing too much objective control. One of the goals of late has been to make Marksmen work in lane positions other than bottom, right? So why not give teams the option to have Marksmen be optional for once?
would all love the item.