So, I've been playing Teemo for a while, got him to mastery 5, and this is what I think of him
TLDR: He feels SO WEAK.
Long version:
So, Teemo has very poor dueling capabilities, even when ahead a bit. His main source of damage are his auto attacks and blinding darts. I have complaints for each ability, with the exception of his shrooms, so I'll compile a list of what I think for each one.
Blinding Dart: The damage on this ability is neat, and so is the blind, and it definitely helps with his dueling potential, the issue is that as you level it up, it's cost increases dramatically. At level 5, it costs 90 mana. Given Teemo's small manapool, this makes him run out very quickly if enemies are aggressive. Leveling this first also poses the problem with lack of consistent DPS on his auto attacks via Toxic Darts, which in turn, makes his damage even poorer.
Move quick: The speed boost on this ability feels like almost nothing. It's cheap yes, but the fact of the matter is that literally every other champion that has a movement speed buff, has it stronger at level 1. Skarner, Miss Fortune, Sivir, Udyr, are just some names. Keep in mind, Teemo is a SCOUT. He should be able to move a bit quicker. This ability is always the one I level last, because even if I level it, it's still pretty bad and won't help me much, unless I'm trying to run away.
Toxic Darts: Teemo's bread and butter. My only complaint is how fast the poison damages you. I honestly think that it needs the "Singed treatment" and deal damage every 0.3 seconds rather than 0.5 seconds.
All in all, I don't get why Teemo is so hated. Sure it's annoying to step on a shroom when you're low and die to it, and sure, it's annoying to be blinded. But there are so many champions that are also annoying. Jinx rocket, Ezreal banana, RENGAR, the list goes on. Teemo's shrooms also have some pretty good counterplay. Pink Wards and the red trinket provide excellent coverage, and also can be used to bait Teemo himself.
So, now that I've gotten my complaints across, here's what I think can be done to make Teemo a bit more playable:
Passive: When Teemo comes out of his Stealth, make his first 3 auto attacks fire in rapid succession (Like Jayce) and also deal a bit more damage for those 3 attacks only. Then, alongside those 3 attacks, he gets a medium movement speed buff. The changes here are to capitalize on his scouting. The reason I wanted him to have a Jayce-like buff rather than a flat attack speed buff is because usually, he'll only get a couple auto attacks in before the enemy simply runs away, or turns and tries to fight back. The movement speed buff is to allow Teemo to run for the sake of re-positioning after remaining still for so long, like a scout. This more than likely won't kill anyone and only serves to rough his enemeis up a little, unless Teemo is really ahead.
Q: Lower the mana cost to something like 60 at all levels. Maybe even have it scale up to 75 if 60 at all levels is too strong. The change here is so that Teemo doesn't run out of mana after just a few casts of this ability.
W: First, remove the tiny passive movement speed. It's practically unnoticeable. My idea for this, is to make it a toggled ability to run. Similar to FPS games. While active, Teemo will move a lot quicker, but will also periodically drain his mana. Let's be honest, the live version of Move Quick is pitiful. This change will prove to be a lot more efficient, and give a more "scout" feel to Teemo. Teemo's main purpose isn't damage, or assassination, it's map presence, which unless he's ahead, he doesn't have much. In his current form, he simply isn't fast enough to be able to move around the map unless he has boots of mobility, which he really shouldn't be building.
E: Make it so the ticks apply every 0.3 seconds, that is all.
Why buff Teemo? Because, Teemo is going to be getting a very neat skin soon, and it would be a pain to have so many people wanting to start maining their glorious satan, when he is so weak. Also, our lord and savior lord satan deserves some love.