I'm so happy to get old Kog back! I'm ecstatic about this direction and I've missed Kog so much! Even though I never even got out of Gold with Kog, I had more fun playing him than any other champion and loved his power curve too. I felt like I was playing some adventure game with me and Koggy against the dangerous and unforgiving world of Malphite ults., Lee Sin Qs and Fizz Sharks. It was treacherous but like a Phoenix (maybe this is why I love the butterfly skin), I could sometimes rise from the ashes and carry the game if given enough time and accurate ults. from a good position. I have a counterpunching and defensive style which he suited perfectly (his passive fit that too).
I had over 600 games on Kog in S4 - mostly mid with some ADC sprinkled in (my kiting sucked but it felt great whenever I could pull it off). I had probably even more in S5 considering some new accounts I made for him. I played him almost exclusively mid at this point and looked up to guys like Knoch who were amazingly good with ult. prediction and just knowing the limits perfectly. I was still stuck in Gold and made huge macro mistakes but even in terms of predicting with ults., it was a huge difference between me and Diamond players. I felt there was always so much to improve on with him and so many builds to experiment with but I didn't feel overwhelmed by it. As a poker player, I also loved the psychology of Kog's ult. and seeing how they juke it for the first few minutes only to then predict their movements after.
When the rework came, I felt that the styles for him were extremely constrained and despite a good 200+ games as ADC, JG and Mid, I couldn't recapture the magic. He just wasn't as fun and rewarding as my old beloved AP Kog so I gave him up. I've tried every build mid and I just think he absolutely sucks and doesn't fulfill any niche anymore. It feels awkward and too squishy to build him like bot lane Kog while also just being no fun for me even if it were strong. Poke AP Kog with Rylai's and Liandry's just seemed pathetic for poke with dft and even if you could get them lower with E and maybe some Sejuani ult. etc., it didn't feel worth it. Why would I pick Kog mid for poke when there's Xerath, Ziggs, Vel'Koz, Varus etc. who all have better poke while also having stronger lanes and more teamfight control. If you want a cleanup champion, Katarina and Kassadin can do it better while also spiking faster, being mobile and having strong roaming. I tried Luden's and basically full AP with a bit more success but it seemed really cheesy and still not that fun. You're basically hoping to land E or you're totally useless and you are squishy without an escape and almost no crowd control. Again, it doesn't seem congruent and I'd rather pick Annie, LB, Viktor etc. if I want a burst champion. Moreover, none of these builds really felt that powerful late game unless I was very far ahead of the game. AP Kog used to be a vulnerable but terrifying poke/DPS (till he ran OOM anyway) hypercarry with a slow power curve and there's nothing else like it right now. Ziggs is what I tried to replace him with but it's still a very different power curve (Ziggs is no hypercarry), style and strengths/weaknesses. I like Ziggs but AP Kog was still more fun to me and more of the cat & mouse game I wanted to play (starting as the mouse and later becoming the cat! :D).
Edit: Just noticed that no ult. changes are planned. I don't see why. Even just before he was reworked, Kog was not a problem in either competitive or SQ. His ult. did punish poorly drafted comps. without engage but he was definitely not dominant in either lane and never even picked mid in competitive after some MS nerfs. His SQ banrate was also never high even when he was OP in competitive because he had clearly defined strengths and weaknesses as well as ample time for the enemy team to punish his power curve. It felt fair.
If Kog's ult. isn't effective as poke/DPS, he just has no place mid. He no longer has a niche there despite having one for 5 seasons and many original Kog fans who loved him for that. He went through every itemization change and survived so he was definitely no gimmick.
Even if new Kog will have 50% winrate mid and be considered balanced, he won't be that amazing unique mid lane pick he used to be (long-range poke/DPS) and it'll most likely just mean his numbers are better than for other mids which actually do have better-defined niches. For example, if Kog is picked for his burst, it'll probably mean Annie, Brand, Syndra etc. have worse numbers and that becomes hard to balance since you're trying to fight bad design with number tweaks.
Old AP Kog had a unique role as the only hypercarry poke + DPS artillery (AKA from the skies not a boring line skillshot) champion and also the riskiest one (weak laning, needs levels & items, cooldown + mana gated, almost no CC, no escape, squishy). Nothing else was like him then and since. Xerath is a surgical champion with good pick potential. He's much safer, has good CC, decent laning and more burst but far less DPS and range (since his ult. has a significant CD unlike Kog's). Vel'Koz is a teamight all-in/poke champion hybrid with good tank shredding and excellent burst. He has a bit of everything but still isn't really a DPS poke champion and of course he'd be broken with his CC if he were to become one. Kog was already unique both as an ADC and mid.