Kog's Innate is really strong, it just doesn't fit with his identity
Since some people may disagree, we'll start with the 'really strong' part.
At level 18, Koggy's Innate deals 550 True Damage in an AoE should the enemy team manage to kill him. Let's compare that to Corki's old innate, 10% of his total AD in True Damage OnHit. With the standard Kog build (
) you're looking at approximately 200 attack damage, or 20 True Damage OnHit. In order to deal 550 True Damage, you would need to attack a target 28 times.
Now, just between you and me, Kog is typically going to be killing things in far fewer than 28 basic attacks. So in other words, Kog's Innate is really strong.
It does not, however, fit very well with his kit. Koggy is more or less the ranged equivalent of Darius. If you let him sit there attacking your team, you're all going to die. So the best course of action? Dive the squishy guy and kill him! Which, of course, triggers his Innate, which may or may not mean you get blown up by him when he detonates. Instead of an Innate that punishes you for playing against Kog correctly, he should have an Innate that rewards you for playing against Kog correctly.
To that end, I want to suggest a new Innate.
Insatiable Void: Each time Kog'Maw hits an enemy champion with a skill or basic attack, he gains a stack of Insatiable Void. Kog'Maw's basic attacks deal (small percentage of his total AD/AP) True Damage OnHit per stack of Insatiable Void. Insatiable Void lasts for 4 seconds and the duration refreshes with each new stack added.
End result? If you leave Kog alone and let him ramp up, he's probably going to eat your team. If you blow him up right away, then he's no threat at all.