My prediction: in the first couple of weeks he will be pretty shit. People say "oh but he's just like he was back then <3" 1. no he isnt because his base stats are still shitier than before. For those who dont remember from 5.22 when kog maw was reworked:
BASE HEALTH: 546 ⇒ 517 HEALTH GROWTH STAT: 87 ⇒ 82 BASE ATTACK DAMAGE: 52.04 ⇒ 54.46 ATTACK DAMAGE GROWTH STAT: 3 ⇒ 2.41 BASE MANA: 298.88 ⇒ 322.2 MANA GROWTH STAT: 41 ⇒ 40 I disregarded Attackspeed stats because you can look them up on the 6.19 patch notes and they have changed a few times since 5.22. source: http://lol.gamepedia.com/Kog%27Maw_-_The_Mouth_of_the_Abyss#5.22

concerning ad kog: trinity might be nice on him but then you still have to spam ult and that drains your mana really quickly without giving you even acceptable dmg before 50% even if you hit it. which means that youre probably through 1/2-3/4 of your mana after youve killed one single guy. yay.

concerning ap kog: i dont understand people who say their ap kog will be back. utter bullshit. ap kog will be even worse because he doesnt get the 50% cast time reduction he gets with w on now. so youll be left with a burst mage that cant burst.

1. what people also dont seem to see is that while kog maw will be reverted, the rest of the adcs will stay as strong as they are now, especially jhin will outclass kog maw in almost everything there is to outclass: more utility, more burst, more range with ult and w, even more mobility with the crit speed up and an immensely better laning phase. all that for the trade of having a bit of a harder time against full tanks. hell i think even vayne with the recent buffs will easily beat kog maw in laning phase. But thats just the initial things that i think will be happening, over the long run, if riot keeps their promise and are looking for buffs, if needed, then he'll be easier to balance from there. Maybe give him the zyra treatment and give him an actual combat passive instead, maybe also an ad scaling on w, then he'd be fine in my opinion.