It's high time we bring back Madred's Bloodrazor
Now, to be fair, it's pretty obvious why this thing was bad previously. It was largely a trap option on ADCs and sorta okay on Fighters. But the %Max HP shred was just a fun ability and having it back would be entertaining.
So, that in mind, I've got three ideas for how this could be worked out. One is an ADC focused item, one Fighter focused, and one for AP champions who happen to like auto-attacking. We'll roll with that last one first.
AP Marksmen could potentially be a thing, but their problem is that they've got a pretty glaring gap in their itemization. Having one IAS/OnHit item just doesn't cut it. The fact that they have issues with eating objectives as quickly as an ADC hurts, too. On top of that, they don't really have any way to effectively itemize for sustain. So it's Madred's to the rescue!

Grants IAS, AP, spellvamp, and the following unique passives. UNIQUE Passive: Your basic attacks deal damage equal to 3% of the target's max HP in magic damage (plus 1% per 100 AP). This passive effect can damage towers. UNIQUE Passive: Your OnHit effects now count as spell effects.
Next up? ADC version.

Lol, double swords. This item grants movespeed, IAS, Crit chance, and the following passives. UNIQUE Passive: Your basic attacks deal bonus magic damage equal to 4% of the target's maximum health. NAMED Passive Phantom Hit: Your Critical Strikes apply OnHit effects twice. This passive DOES NOT stack with Sated Devourer.
And finally the Bruiser version.

Solid all around defensive stats, IAS, the Cowl passive, and the following unique passives. Named Passive Cold Steel: Your basic attacks steal 15% of the target's base attack speed. Unique Passive: Your basic attacks deal bonus magic damage equal to 4% of the target's maximum health.