I think it's time to look at Kog'Maw's passive

Now that Zyra has lost her on-death passive, I think it's time to look at Kog'Maw. The fact is: If you're doing well on Kog'Maw, he doesn't have one. If you're doing poorly, it's still unreliable in the mobility-heavy present state of LoL. So here's my train of thought:

Icathian Instability

While out of combat, Kog'Maw slips out of phase with reality, ghosting and gaining increasing movement speed over 4 seconds up to 25%

Upon taking fatal damage, Kog'Maw rapidly gains double his normal movement speed increase over three seconds before exploding one second later for 125 - 550 true damage.

This solves what I consider to be the main issue with Kog'Maw: Getting from point A to point B. There are many times when I have been unable to reach a skirmish in time due to just how slow Kog is, or I've struggled to keep up with a mobile teamfight. Increasing Kog's out-of-combat movespeed would help nudge him along. The extra 10% movespeed on his explosion is intended to deal with high mobility targets that are just slipping out of reach too easily. He gets faster, and he tops out a little sooner. Also note that the phasing speed boost is out-of-combat, taking or dealing any damage instantly drops him back into phase.

Thematically, Kog'Maw is a creature that forced his way into Runeterra by complete accident. He is too dumb to understand where he is or how he came here. The idea of him phasing out of reality is based on him chasing after his next meal, instinctively bending reality to get to it faster.