Riot didn't take into consideration that the 'revert' on Kog kept seven nerfs compared to where Kog was before the pre-season.
- Base HP
- Scaling HP
- W mana cost
- Ult vision duration
- Ult mana cost
- Ult stack timer
- Ult poking power
If it were up to me? I honestly think Kog could use a full VGU. His kit is an absolute mess at this point. Gameplay Hook: Corroding enemy resist and growing from a weak little Void caterpillar early game into an absolute monster late game.
Passive - Acidic Bile: Kog'Maw's basic attacks and abilities apply a stacking debuff for four seconds that reduces the resistance of enemy units by (amount).
Q - Caustic Spittle: (Cooldown reduced dramatically, damage adjusted. Attack speed bonus removed.) Kog'Maw fires a missile that deals magic damage and applies the Corroded debuff to enemy champions hit for four seconds. Corroded causes Acidic Bile stacks to multiplicatively shred (percentage) of the target's resist with (Passive amount) as a minimum value per stack. Successive applications of Caustic Spittle increases the percentile shred value.
W - Living Artillery: Passive: Kog'Maw's basic attacks deal %Max HP damage OnHit.
Active: Kog'Maw gains significant bonus attack range on his next basic attack and applies OnHit effects twice. This ability resets his attack timer.
E - Void Ooze: (Level 1 cooldown increased dramatically, scales down with level) Kog'Maw launches a slime ball that deals magic damage to enemy units hit. The slime ball leaves an ooze trail that slows enemy units by 50% and applies the Grounded debuff.
R - Bio-Arcane Barrage: Passive: When skilled up, Kog'Maw enters a chrysalis for 4 seconds, becoming helpless. Upon emerging he increases in general ferocity, boosting his base attack damage, base attack speed, and flat resistance shred.
Active: Kog'Maw gains (amount) bonus attack range and doubles the strength of his %Max HP magic damage for the next six seconds.
So what have we got here? An evolving champion model that grows up as you invest points in Kog's R. Consider the voidlings we read about in Diana's lore. They started off as white, spindly things that gradually became armored purple monsters. Same general idea with Koggy. He would start off more or less like he does on Live, but as he grows up you get a bigger, meaner, purpler Kog. By making his base attack speed and base attack damage scale with his Ult, it would allow his strength at various stages of the game to be more finely tuned.
Improving the utility of his E and focusing more on his resistance shredding would give Kog some power outside his raw damage output. Which is important, since trying to balance Kog purely around raw damage hasn't exactly gone well. The amount of damage he needs to be able to deal in order to be an attractive pick over other late game ADCs who ALSO have mobility or utility (or both) is frankly unhealthy for the game. Kog needs something else to offer apart from sick deeps. A grounding E would help keep foes at arm's length (while doing very little against someone already on top of him) and heavy resist shred would be helpful on teams with low damage (protect the Kog) or teams with heavy burst for dealing with a beefy front-line.
The new W active mana cost would work quite similarly to Kog's current Ult. It would be a low cooldown ability encouraging Trinity spam. However, it's mana cost would ramp up, requiring investment in mana or conservative use. The range bonus from Kog's W and R would in fact stack, however given the limited duration of the new Bio-Arcane Barrage (and higher cooldown) using it just to poke with W-enhanced auto-attacks might not be wise.
Sadly, this would be removing Kog's current Ult completely. But... honestly the thing is a shell of its former self at this point. Kog's kit was previously centered around Living Artillery, but Riot's insistence that it's poke pattern is 'unhealthy' and 'painful' makes it unlikely we'll ever see it again. Better to just cut it off and take Kog in a new direction.