Kog'Maw Revert Alternatives - Make his power conditional
I must say that I consider the Kog'maw reverts somewhat uncharacteristic of Riot's balance team and their ethos. When a champ has too much power, Riot will try to look for ways to attach conditions to the ability.
While I do sympathize with how much his W synergizes with on-hit items and how hard it makes balancing those items, I think the biggest reason contributing to Kog's 'Toxicity' is the lack of tangible weaknesses other than immobility and squishy-ness.
Some (somewhat radical) ideas for Kog's W that I might suggest:
While W is active, Kog'maw applies Self-CC (slow or root), either for partial or whole duration. (We often observe this on live anyhow)
Before activating W, Kog'maw goes through a brief channel / cast animation (thematically, this could be him eating or processing something before spewing it back out)
This would really reinforce his 'immobile carry' status by making him extra-vulnerable, and would force the Kog to search for opportune, safe moments to pewpewpew.
While W is active, designate an 'inner range' that Kog'maw cannot attack enemies in - when an opponent manages to get into melee range, Kog cannot target them with his auto-attacks.
When W is activated, designate a (wide) Cone in front of Kog that he is limited to attacking in - Kog cannot target enemies outside of the cone (In a very 'Bastion'ey sort of fashion).
These changes would really weaken Kog's 1v1 dueling power, particularly against mobile enemies, while really cementing his reliance on peel from his teammates and urging the Kog'maw to look for opportune moments.
Decrease the Duration of the W and/or Increase the Cooldown between W's
When the Kog'maw chooses to activate his W, he is indicating his commitment to the fight, and opponents should be able to better capitalize on it's downtime.
Reduce (or remove) the Base On-hit damage Kog'maw gets by ranking up his W.
This would serve to reduce his damage against squishies and lessen his mid-game strength. (Though I'm aware that the current value isn't much at all)
It's a shame for something so iconic to be lost - this 'living machine gun' identity is something that does set Kog apart from not just Marksmen, but the entire cast of Champions, and it's definitely something a good deal of us will miss.
I'm sure Riot could pull something else out of their hat other than a revert.
