So I played a couple matches earlier tonight on the PBE as Teemo, who already has a powerful on-hit in his E. Runaan's Hurricane is a great item, and synergizes a bit too well with Teemo and I was curious why on-hits carried over with the item.

As an example, I built Teemo with Sorc Shoes, Malady, Rabadon's Deathcap, and Runaan's Hurricane. Quick in-head math is telling me that I'm sitting around 200 AP after masteries (I had no AP from runes, only spell pen), which is pretty nice. With Teemo's E (Maxed first btw) having a .4 ratio and Malady now having a .1 ratio on all auto-attacks, I'm looking at over 100 magic damage getting added to every auto-attack without the base damages from my E or Malady, 170 when you factor those in. Also, I will have at least 1.7 AS (Level 9 with 120% AS added, gets higher as I continue to level and I'm not likely to have these items before level 13 even). Also my natural attack damage is roughly 85-100 around that point.

The damage output for this pretty simple AP/AS build would be strong by itself if it was all focused on a single target (255-300 on the primary target, with a 1.7 AS); instead I'm doing that much to the primary target and then only about 40 damage less to the secondary opponents, since the build is doing damage almost entirely via the on-hit effects. This seems a little outrageous since it essentially obsoletes every other Teemo build through the use of this single item; I would have higher burst output with a full AP build of Teemo, but single-target only in a fight and a low AS, while an AD Teemo with this item wouldn't have nearly the damage output on the secondary targets.

Why does Runaan's Hurricane apply on-hit effects, especially stuff like Teemo's poison? It is a great item on Teemo (An /AMAZING/ item), but I am worried that it's so good that it is choking the build option of pretty much any champion who can buy it. I faced a Twitch earlier tonight that used Runaan's in conjunction with a Blade of the Ruined King, and watched as he melted our entire team in fight after fight while life-stealing thanks to the bonus effect of the Blade.

I think what Riot had in mind is an item for ADC's to sit in the back of teamfights and lay as much damage to a group huddle in a team as possible.
Alot of the new items also have new on-hit effects so it encourages item synchronization.
It scales of bonus AD so in AP Teemos case, enemies should be building MR to counter him, Hurricane wont be hitting as hard as you would think, just laying down on-hit effects.
IMO It's not close to touching a level of OP & the On-hit application isn't so bad as to deserve a change.

*I'd like to add that change is good for this game. After a while the game becomes bland and stagnant. I think it'll do the game some good to see champs (like u mentioned, Twitch) that are never played arise from the ashes of unviability via new item builds and strategies.*

Which is why I'm asking why the on-hits are getting applied at all, since the item was clearly meant for the adc on the team; Teemo makes better use of this item than any ADC as a result of his natural on-hit already being incredibly potent. Teemo is doing more damage with this item than any ADC would be doing for the same amount of gold, and he scales the damage on it as a 55% AP (E, Malady, and the Spellsword mastery) rather than 50% AD. Considering that AP is cheaper than AD (+80 for 1600 instead of +45 for 1550), Teemo is scaling much MUCH faster than an ADC will be scaling, while still retaining high damage on both his Q and his R.

Yes, most on-hit items are magic damage and thus can be countered by MR, but arguing that MR counters this build is akin to saying that armor would shut down an equivalent AD version of the build; true, but missing the point of the initial argument. On-hit effects are overkill on an already powerful item, leading to an abuse case like Teemo where he has DPS similar to an ADC even well into late-game while having more potent AoE and more damaging harass.

good.... they could use some dusting off from the shelves of obscurity and maybe will will see more AP teemo mids!

One could argue that on-hit hybrids like Teemo need more help than AD carries in the first place given that their representation in high level play is almost nil.

That isn't to say that this item is useless on all ad carries. Twitch makes good use of it (although he's pretty squishy and might be better off with a defensive item or two); Kog'Maw seems downright scary with this item as well. However, AD carries have so many good items at their disposal now that Hurricane has to be "worth it" to even be considered.

Xyp mentioned in another thread that Teemo's poison might be a little too strong on Live at the moment and I can confirm that he is, indeed, very strong on the PBE with hybrid penetration runes and a Hurricane.

Remember that it will help those ADC's who are underplayed on live server right now.
Ezreal, Corki and Vayne can't make use of it. Graves can't get much besides of his cdr of his AS-steroid.

On the other hand, Caitlyn, Varus, Kog'Maw, Twitch make good use of it. Maybe Ashe as well (though this is more for utility than for AoE dmg).
If it's just a problem with Teemo, you can nerf his poison accordingly.

Also take into account that Hurricane prefers a completely different build, since it doesn't applies crits. You have to choose between extremly high single target dmg (IE/PD/BT (or BC/LW)) or high AoE-dmg.
It leaves a lot of ADC's with a new option.

I'm pretty sure all the mentioned champions can build hybrid builds as well, if they build Hurricane. Just try it.

Because letting it apply on hit effects lets it do really cool stuff, and that's what makes the item so fun. Teemo's poison probably needs to be nerfed anyways, so it's not that big of a deal. I'm more worried about Malady on him.

Teemo's damage is a DoT right? Every hit he gives to enemies refreshed the DoT. The initial damage isn't that different from a 100-200 autoattack plus the malady's bonus magic damage.

Because if it didn't apply on-hits it wouldn't be very good? The only ranged champs I expect to see it on are ones who have on-hit abilities or already benefit from on-hit items. On a regular ADC I'd just rather an item that helped me blow up my main target faster than getting moderate AoE damage.

It isn't really an AD item. It's an on-hit item, of which we needed better itemization.

Teemo will get nerfed.
They either make his poison to be on the last target he hit (no more multi dots) or reduce the AP ratios by a lot.