Feel free to downvote this post and everything, because I know we all love (to hate) Teemo. ;) This is only my opinion through experience, don't expect me to back up my post with hard 'facts'.
I think that Teemo's E is too weak in this current meta. Its AP ratios are too low when dealing with full tanks / bruisers (e.g. Malphite / Zac) that run a full MR rune page, and that rush Spirit Visage / Hexdrinker on first back.
I know they are called tanks for a reason, but when I have to go AD and build Crit + Blade of the Ruined King + Last Whisper item just to deal with a 220 MR 3500+ HP Olaf because going Ability Power means I deal pitiful damage even with a Void Staff, there might be a problem.
However, I do realise that raising those ratios could potentially make Teemo OP.
I propose to make his E work like somewhat like old Fizz's W, dealing:
10/20/30/40/50 (+30% AP) magic damage on impact, and 6/12/18/24/30 + 2/3/4/5/6% of the target's ** current** health as magic damage per second for 4 seconds.
Or this one, which I like more, dealing:
10/20/30/40/50 (+30% AP) magic damage on impact, and 1/2/3/4/5% of the target's **max** health as magic damage over 4 seconds, stacking up to 4 times. (4/8/12/16/20% max HP over 4 autos at full stacks).
At full stacks, the target becomes _poisoned_ , and is slowed by 40%, losing 30% of their Max Health, Armor, Magic Resist, Attack Damage, and Ability Power for 4 seconds. Poisoning cannot occur on the same champion within 12 seconds.
It wouldn't be THAT overpowered vs squishies, and it would feel more fair vs the supertanks that I face often this meta.
As for his W...his W is a weird ability. I don't really know if I would change it at all, but if I did I would raise the MS on its active by about 5%, scaling with level. Also his HP/level could use a slight buff (maybe by 200 or so at level 18).
TL;DR: I know that Teemo currently is in a decent spot right now, but I feel that he is countered too easily by most toplaners if they build (and stack) early MR (especially with the new Courage of the Colossus mastery). And with his short AA range he is wrecked by many AP midlaners if you cannot outplay. So make the DOT portion of his E deal % health damage over time.