I hate Teemo. Don't you? Laning against him feels like you stepped in something. Something smelly that stains. He has unrelenting ranged harass, and he is hard to catch or gank. If you can't catch him, pin him down, and also beat his all-in, then the lane is completely one-sided. You can't survive him because his zone range is too long. You can't gank him because his positioning is too safe. You either leave lane forever or die repeatedly.
What keeps Teemo in check is his lackluster late-game. We saw much more of him those few months when AP Teemo had strong mushroom-based map control. He fell out of favor when his late-game was nerfed.
But Teemo is back and stronger than ever. Not in person, but vicariously through a new squadron of slippery ranged top laners with solid late-games: Elise, Jayce, Kennen, and until recent nerfs, Nidalee. They harass relentlessly and escape effortlessly. Losing lane to them snowballs completely out of control, with the victim feeling completely helpless. Many champions lack the kit to handle them and lose at champ select.
This is a problem. These are popular picks with no exploitable weaknesses, invalidating whole classes of champions, and generally making their opponents feel awful.
Teemo was brought into line by nerfs to his late-game. Nidalee was brought into line by nerfing her all-in. What to do with Elise, Jayce, and Kennen? I would look at their escape cooldowns. Currently, there's no value in forcing the ranged champion to pop its escape. The cooldown is so short that the harass begins again immediately. Increasing these cooldowns would make the counterplay meaningful. The downtime creates a window of opportunity. The ranged champion has to either accept the risk that you can catch and finish it, or back off for a while and let you farm.
Kennen's Lightning Rush has a cooldown of 10/9/8/7/6. I would nerf it to 14/12/10/8/6, or even 16/13.5/11/8.5/6. With Elise and Jayce, their escapes are based on their transforms. These two champions are a much more blatant balance issue, and deserve a more severe nerf. I would increase the cooldowns on their transforms from 6 seconds to 12 or even 15 seconds. If this seems harsh, remember that those transforms are as powerful as ults, and deserve cooldowns to match.
With these nerfs, these Modern Teemos will still be strong. But now forcing them to pop their escapes will put some fear in their hearts and give their opponents some badly needed breathing room.