Kennen ADC has seen LCS and competitive play before, most notoriously by Fnatic's Rekkles, during the 2015 World's Championship. After Season 6 started with the ADC rework and buffed ADCs, Kennen just seemed to be forgotten with the likes of Mordekaiser bot. I believe that the new mage update, coupled with the Runaan's Hurricane change at the start of Season 6, has saved ADC Kennen. It just needs to be presented to the public.

Although Kennen's Mark of the Storm stun duration and [W] Electrical Surge active range got nerfed, his [R] Slicing Maelstrom gained consistency, something that is vital for an ADC's CC to be effective.

Kennen's gameplay revolves around the minigame of tagging the enemy laner(s) with his Mark of the Storm passive. When he gets 3 marks on a target before they diminish, his target gets stunned, with each subsequent stun having a diminished effect. Use the stun to open a window of opportunity in which you can auto-attack the target for free, without fear of retaliation.

In teamfights, you want to zone enemies away from yourself and other high priority targets with your [R] Slicing Maelstrom. You need to be able to activate your Mark of the Storm both on priority targets to diminish their DPS and on Tanks and bruisers to lock them down and protect your backline. You want to be auto-attacking as much as possible to build Fervor of Battle stacks, and whittle down their frontline before rushing on top of their squishy backline.

In terms of damage and scaling, ADC Kennen is most comparable to Twitch.
In terms of crowd control, ADC Kennen is most comparable to Ashe.
In terms of mobility, ADC Kennen is most comparable to Quinn's Rank 2 Ultimate, but for 2 seconds and no channel.
In terms of range, ADC Kennen has an Attack range of 550, which is equivalent to Corki, Draven, Ezreal, Jhin, Kalista, Miss Fortune, and Vayne.

Kennen provides a very safe laning phase and can wreak havoc in a chaotic teamfight with his AoE damage and hard CC. I personally find late-game hyper carries to be very fun, as well as cheesing people with on-demand, reliable Hard CC.

I do not claim Kennen ADC to be broken in any way, shape, or form. I believe he is in a very balanced state and has a relatively high skill floor because of your need to manage Electrical Surge charges. I only wish to reintroduce him as a viable option.

Interestingly enough, a professional player, brTT from Brazil's paiN Gaming has played a very few amount of games on ADC Kennen. His match history is below.

http://br.op.gg/summoner/userName=paiN+brTTrxrx

Because Kennen ADC gives up raw DPS for utility, Kennen does well when your team has a secondary form of sustained DPS or DPS that needs sufficient peel in order to fully have an effect, so that Kennen can peel for himself while simultaneously peeling for the other DPS dealer. Examples of this are Quinn, Kindred, Gangplank, Darius, Katarina, Master Yi, and Riven. If your team already has a whole lot of peel, but not that much damage, then Kennen might not be the best pick for your team. Keep this in mind.

TL;DR As ADC Kennen, you want to max your skills as [R]->[W]->[E]->[Q].

Your [Q] Thundering Shuriken, has no AD ratio. It will only be used for its Utility to proc your Mark of the Storm. You take this at level 1 because it can help you push the wave faster to hit level 2 first, and then Max it last.

Your [W] Electrical Surge's passive has your only AD Ratio, so you max that first. You use the Active portion of this spell solely for its utility to proc Mark of the Storm.

Your [E] Lightning Rush gives Kennen a gigantic movement speed steroid at the cost of not being able to Auto-Attack. Learning to use this efficiently is what separates the good ADC Kennens from the bad ones. Use this to escape, close gaps, initiate, chase, or to just get from place to place faster. You can even use this to tag a Mark of the Storm if you're in a pinch. The energy cost and cooldown are both reduced with rank, and you get free Armor and MR, so max this second. [Note: Remember that you get energy back if you hit something with Lightning Rush]

Your [R] Slicing Maelstrom is Kennen's best ability that can single-handedly win him a teamfight or a game. It is just so versatile and useful on an ADC. It will win you 1v1 and 2v2 confrontations if you are able to connect it and auto throughout its duration. Use this in conjunction with Movement speed buffs (Such as Youmuu's Ghostblade, [E] Lightning Rush, or an ally's ability) to be able to reach targets easier. You can pop Slicing Maelstrom if a tank or bruiser tries to dive on top of you during a skirmish or teamfight (a fed assassin will just straight up kill any ADC without Guardian Angel, so don't expect it to do much there). You can even use it to escape a goon squad 5 man botlane gank by being able to tag multiple targets with it and running away. Take a point in it whenever possible.

- Runaan's bolts will not proc Kennen's [W] Electrical Surge passive. They will, however, charge it up so you can build up your passive 2-3 times as fast, which is incredibly strong.

- Kennen's [E] Lightning Rush grants Kennen 100% of his base movement speed as bonus movement speed. This means that if you combine the effects of Lightning Rush and Youmuu's Ghostblade's active, you essentially double the item's effective movement speed to 40% as well as the other %movement speed items (Zeal) you have in your inventory. Note that [Lightning Rush] will have no effect on Flat Movement speed buff via Boots.

- Kennen's Slicing Maelstrom will persist through Guardian Angel's psuedo-death animation.