Conclustion: Passive: Keep Q: Passive increase base on missing health. Active becomes ar/mr reduction. W: Have a health cost, reduce cooldown. E: Linear skill that damage and slow upon hitting. R: Keep
Basically, I made Kog'Maw more offensive, risking his life for more power (in the lore, Kog seems to has an unlimited hunger that he eats without thinking). Q passive makes more sense with the theme. Buff the passive so I reduce the active. W: cost health, to increase the Q passive benefit. E: I want to keep Kog save from gank. By nerfing the area, the range, the base slow, I reduce the cooldown, so he can "hit and run" more. By increasing the slow when taking damage, he can chase his victim with his ultimate, slowing it.
Change resource from mana to health. Change Q from a buff to a linear skill. Change W to a toogle skill. Change W to cost 1% maximum health and increase while still using the skill. Change E back to it original, with the range nerf, lower ability power ratio and slow.