Kog'Maw
What to Keep:
His playstyle as a true hyper carry is unique for the role and it definitely is a viable option. I do feel his Q-Caustic Spittle is a fine ability and the AS needs to be there to not make his W-Bio Arcane Barrage so overloaded. Together with his E-Void Ooze, his three main abilities would be fine to keep.
What to Change
P-Icathian Surprise: is easily one of the most anti synergistic, archaic, and overall worst abilities in the whole game; I mean, it is an ability that activates when the hypercarry Long Range AD dies, something he should avoid at all cost. This means that a whole new Passive would be needed. I would call it, Icathian Gluttony, and it would make it so whenever Kog'Maw kills or assists in a kill of an enemy covered with "Goo"(which is a new keyword whenever Kog hits enemy champions with Q, E, R, or while W is active), he would receive a "Protective Skin" where if killed, would be put to revive in an invulnerable Cocoon after 2 seconds. Wordy I know, but basically if Kog hits an enemy with an ability or with W active, he would be protected from death for around 30s(how long the protective skin lasts) which would then go into an internal cooldown. I think this DOES fit his playstyle way more as he would not just die and explode doing damage, but instead have a protection if focused.
R-Living Artillery: I would keep the active of this ability but instead of the debuff from spamming it, I would give it a higher cooldown but with a catch. It would have a new Passive where Goo'd enemies would be burned for 1 sec after being hit by Living Artillery; which could stack up to a max amount of damage. I love the idea of changing his R from a poke ability to a Secure type ability since he should not really be a poking mage and should instead be a gluttonous caterpillar who wants to "prep", hence the Goo, his meal before digging in.