AP kog was my favorite champ. pre-rework and just doesn't really work anymore. His late game fantasy is still there somewhat but he is nowhere near as strong as before in my experience.
The mana costs are up on both W and R, the base damages are lower and he scales worse on top of it. His ultimate used to offer massive artillery support in teamfights and now it's basically a clean-up tool. His laning is substantially weaker too (you can't just cs with W due to mana costs) and I just don't see almost any upside to him.
The way W works now isn't fun or interesting at all. You basically gimp yourself so you're unable to kite and do less damage only to spam more autos. How is that better? I'd rather have a meaningful attack and more control. He doesn't even do more damage. If you count level 18 base damages for W, a 5 AS Kog will do less than the old Kog which had a free W. 6% on hit magic damage down to 2% is huge and it has worse AP scaling on top of it. I tried a Nashor's Tooth Gunblade build and felt almost useless most of the game. You just don't have the scaling/safety to come on-line in a reasonable time frame.
I'd at least tolerate the W change if his ult. was left alone. However, his ult. is also clearly worse overall despite having nice scaling when they're low HP. You can run the math but it's basically always worse unless you have some massive initiators like Malphite and Annie to soften them up heavily before (at which point you'd likely win regardless).
I feel like they deleted my favorite champion. He already had many interesting build paths, styles and flexibility of roles. He wasn't popular but he had his niche and rewarded mastery. The way he is now feels pretty terrible almost every time I tried him. I don't mean that I can't win but rather that my influence is drastically reduced and I feel like I'm harming my team by picking him. Before this rework, I could at least rely on my late game to turn around game with my poke.
My proposal: His ult. damage changes and increased mana costs make a poke mage style mediocre at best. His kit suggests a hybrid build path with Nashor's but that is still bad. What he needs is a much stronger W so that this style can work well. I'd suggest 3% base with 2% AP scaling.