It's good to hear there is going to be further work going into Teemo's E, but that's far from the only problem that plays into what I consider an overall dissatisfying update.
The only build I feel still works is On-Hit, because so much of his power has been put onto the new passive. Q is weaker, but empowers passive. R is weaker, but empowers passive. E is a mess, but gives you an AS buff that will empower passive. W is still irrelevant to everyone's dismay.
TL;DR - The passive is really strong, but the cost to his overall power makes on-hit seem like the only viable strat. Q nerf means he can't assassinate well. W sucks. E shouldn't have an inbuilt detriment like a slow or footprints or any damage reveals, but if there must be such a mechanic put it on W as a double edged sword. R bouncing shrooms is a joke. 5 shroom cap is a placebo; give us a lower recharge or longer duration.
Breaking it down
Passive: Poison on passive is nice if solely for the fact it gives me flexibility in ranking other spells. Gaining poison damage with levels is certainly nice, and there is no denying that the 50% amp is really strong. To the point that I fear it crowds out his power budget, forcing his other spells to be weak to the point that only On-Hit builds still work. Th passive has more base damage at later levels, and the instant On-Hit also has a lower AP ratio so that doesn't help AP or Assassin builds.
Q: Lower base, particularly at higher ranks, and 20% less AP means this is a significantly weaker burst tool. The burst this spell offered is largely what made Teemo capable of assassinating targets from stealth.
Coupled with the inconsistency of the blind debuff leave this spell still feeling lackluster. For example, Ekko's E still applies passive and damage if he is blinded. I believe there are other on-hit effects that still apply during blind, and empowered auto's are not consumed during blind which really sucks. Basically this will slightly reduce incoming damage for a brief duration, but more importantly will empower your passive - again all about On-Hit.
W: This spell is still lame. There is no scaling, there aren't any additional effects, it's really just boring. One thing that the rework has repeatedly been trying to do is find ways to make Teemo's stealth weaker. I disagree with this approach, as I'll get into next, but if it's non-negotiable then tie the detriment to W. If Teemo casts W while invisible he leaves behind toxic footprints in his haste. The footprints will give a clear indicator of Teemo's whereabouts, but allow him to travel faster. The footprints can either apply a some form of debuff/small DoT, or even just mark the target as "poisoned" for the effect of his passive.
E: Obviously this is the most contentious spell, since it's the biggest change, and there are similar spells to look at for comparison. Namely Twitch Q.
First looking at current numbers involved. Teemo can move 700/750/800/850/900 units. Base movespeed translates to 2.12/2.27/2.42/2.58/2.73 seconds of travel. More movespeed (like W) will reduce that duration. He gains 25% vision radius. He gains 20/30/40/50/60% AS for 3 seconds. CD is 40/37/34/31/28s - reduced while in bush.
Twitch's Q lasts 10/11/12/13/14 seconds and grants 10/30% movespeed, which would equate to 3630/3993/4356/4719/5082 units of travel. More than 5 times the range at all ranks. Notably, it's camo, not invis, but Twitch doesn't want to be within the 500 units needed to see him, so that doesn't really upset him much. Twitch gains 30/35/40/45/50% AS, so more early less late than Teemo, but his lasts 5 seconds instead of 3. Twitch Q also gets a hard reset if someone dies while poisoned, and it's otherwise on a flat 16 second cooldown.
Taking all of this in, I think it's pretty easier to argue that Twitch Q far surpasses the currently proposed Teemo E. It lasts longer, has a stronger steroid early that allows for easier snowballing, has a more powerful reset, and provides MS during the stealth. One of the two main uses of Teemo's stealth (and Twitch's) is to surprise an enemy and assassinate them. Twitch has more burst than Teemo, more range than Teemo, and more AoE damage than Teemo (Twitch always gets Hurricane, and uses it better than Teemo can), and his stealth tool, at least so far as I see it, is stronger than Teemo's.
Despite all of this, you are adamant about Teemo having some additional detriment to his stealth. I think this is ridiculous, but if it's the only way for this to work, then as in my above point, tie it to his W. Give the Teemo player the choice between "true" stealth, or "rushed" stealth. If he chooses to use move quick while invisible he gives away information about his location, but the opponent then needs to decide if they want to chase him. While we're at it give, either make E duration based, or distance with a MS ratio; it's pathetic right now.
R: I swear to god if "infinite bouncing mushrooms" is taking up a non-zero amount of Teemo's power budget I will scream. This is the stupidest addition I've ever seen. I hate it more than I despise Dunkmaster Ivern, which I thought I hated with every fiber of my being until I saw this. THE ONLY TIME I HAVE EVER USED THIS WAS IN A MOCK ATTEMPT TO "STEAL" OVER BARON WALL. It's a joke in lane. It's a joke in teamfights. It's a joke when split pushing. And frankly, it's a joke when trying to steal an objective. You would be more likely to succeed a steal by using the blast cone and casting Q; bonus points for using A>Q>A, which would be more damage than the full duration of the mushroom! Instead of forcing me to use two shrooms to "safely" get a mushroom in a bush I know is being camped, give me more than a 400 unit range at rank 1! Almost every dash/blink/ranged CC outperforms 400 units!
For the rest of it: I get that the general community sentiment is that shrooms are obnoxious. I also get that if you build a tiny bit of MR and actually make use of sweepers and pinks then they aren't a problem. If the bottom line is that the Teemo player must be held accountable for the failings of ADCs everywhere and shroom damage needs to be nerfed then so be it. But for all that is good give me some real compensation instead of "4/5 shroom cap at levels 11/16". Give me a lower recharge time! If they hurt so much less let me have more of them! That's that goal of increasing the cap, but the problem is the only time Teemo actually hits the 3 shroom cap is if he dies, if he recalls with 2/3 shrooms, or if he's not playing well! AT ABSOLUTE MAXIMUM this increases total shroom threshold by 2. You could accomplish the same by reducing the recharge by a tiny amount, or extending the duration by 30 seconds even!