Kog'Maw and Sejuani I will take about here:
It may be cool to make the pig riding girl the super slow tank with having Permafrost's (E) slow stronger at max rank, maybe something like 40/50/60/70/80% ... 45/55/65/75/85% ... 50/60/70/80/90% from 50/55/60/65/70% and have her with a unique strength that way, but being able to apply permafrost from ally slows is a nice quality of life touch and W being AA reset.
Base Attack Speed: Maybe something like 0.645 +2.5% from 0.625 +3% be better allowing him to scale well with attack speed items.
Caustic Spittle (Q): Perhaps have the shred 15/20/25/30/35% and increase the AP ratio to 70% from 50%.
Bio-Arcane Barrage (W): Changes are good, perhaps have the duration scale like 4/5/6/7/8 seconds instead of 6 seconds flat more of a thing to try to have him not overbearing in the early game, but allows enough time to do projectile vomit when maxed... you know Kog'Maw things! Anyways just something that can be worth testing.
Living Artillery (R): Finally, AP Kog'Maw may finally make a come back with this being reverted, 10 seconds was waaaay too long to wait.