The real problem with Kog'maw...
Is his passive. 5.22 introduced new passives to Graves, Corki, and Miss Fortune as well as giving love to other marksmen's passives. Passives help define what a champions role is, and they help new players understand the strengths of that champion, both of which Kogs passive do not do Kog's passive does not reward players for A. Positioning themselves well in a fight B. Protecting an allied Kog by grouping around him. So I propose a new passive that accentuates Kog'maw's need for careful positioning and a team to back him up.
New Passive - Daddy's presence Kog'maw gains bonuses for each nearby allied champion (Range - 1300) 1 champion - +10% base health and mana regeneraion 2 champions - +10 movement speed 3 champions - +15 armor and magic resist 4 champions - Other bonuses are doubled
Though these numbers may be a bit high/low I think it conveys the idea well enough. Stick together as a team and be rewarded for it. Offensive hard stats are a no go as I believe it might enable 5 man jungle invades before creeps spawn and that would betray Kog's identity as a late game hyper carry, but each tier should benefit Kog when the time is right.
The first tier is to help Kog through the laning phase bolstering his regenerative stats as long as his support stays alive, if Kog and his support get into a scrap and the support dies kog can't simply limp back to tower and lick his wounds at an increased rate. The second tier is movement speed to help Kog keep up with his jungler and support during ganks. As one of the few champions in the game without a mobility ability Kog has a lot of trouble during ganks often being left behind when everyone else pursues kills. The third tier should be achieved well into the game both teams are beginning to group and Kog is finally able to show off his stuff, but nothing hurts worse than an assassin or an ability power carry absolutely destroying you before you have even a chance to act. This tier simply helps keep Kog from going 100 to splat in the blink of an eye. The fourth tier is not very powerful and it doesn't have to be. When you achieve this tier your team is grouped and if they protect you well you should not need very much internal help.
I think this mock up is a very good starting point for working up a new passive for Kog'maw because he sorely needs one, and I hope that everyone else likes this idea.