I don't know, it just feels like the "VIP nuclear football protect me to win" ADC having a passive that only triggers if he dies is... weird.
Here's a random not very inspired idea that's at least trying to stay on theme.
_**P: Icathian Acid**_
Damaging an enemy champion or monster with one of Kog'Maw's abilities afflicts them with Icathian Acid for X seconds. The affliction causes them to take [10% of the applying ability's damage] as bonus Magic Damage on-hit from Kog'Maw's basic attacks.
Subsequent ability hits will either refresh the acid's duration or enhance it if the new ability hit is stronger than the last.
Kog'Maw's basic attacks will refresh the duration of Icathian Acid on targets, but if the acid expires on a target, they become immune to it for a short time.
When it says [10% of ability's damage], I mean that 10% of that ability's damage and scaling become on-hit magic damage versus that target.
As an example, rank 3 Caustic Spittle does 180 (+50% AP) damage. Landing it would thus grant Kog 18 (+5% AP) on-hit magic damage on that target for a short time.
Might have to adjust some of his other numbers a bit, particularly Bio-Arcane Barrage.
It's a bit more spell-weavy, but that's not entirely unheard of. Kog actually used to build Trinity Force back in the day, after all.
I just think Kog needs a non-death passive is all, and for Nashor's Tooth to not be a bad item for him _(seriously, being the mainly magic-damage-dealing auto-attacking marksman with almost entirely AP scalings, Nashor's SHOULD be good on him like it is on Kai'Sa)_.