A nice, easy way to balance Marksmen Kog
Riot doesn't like AP Kog, apparently. He's been called 'the definition of unhealthy champion design' more than once now. Which is why we're probably never getting his old Ult back ever. Which is sad.
But since Riot isn't ever giving him his old Ult back, the new one really needs some changes to help it pull its weight. Basically, Kog needs a reason to want to use his Ult on enemy champs who aren't already on death's door. And I think I've got a simple way to do that. I don't usually suggest concrete numbers in topics like these, but I think they'll help get across the intent I'm shooting for. Consider them place-holders and nothing more.
R - Living Artillery: Kog'Maw fires a long range projectile that deals 100/150/200 (0.5 bonus AD ratio, 0.3 AP ratio) magic damage in an area after 0.6 seconds. This damage is multiplied by 3 if the target is below 25% maximum health. Additionally, hitting units with Living Artillery applies the Free Food debuff for 4 seconds, revealing them. Kog'Maw's basic attacks against debuffed targets deals bonus magic damage equal to 6/7.5/9% (+1% per 100 AP) of their current HP OnHit. Consecutive casts of Living Artillery within 8 seconds compounds the mana cost.
So Kog would want to tag healthy targets with his Ult to apply the debuff, which gives him access to some hefty %Current HP damage. That extra damage helps him gnaw down healthy targets to get them closer to execute range more quickly. This would also provide Kog with a MUCH needed boost in his mid-game power, because if a Juggermaw comp's Maw isn't kicking in by mid-game when team fights are starting there really isn't any point in running a Juggermaw comp in the first place.