Zombies.
As most people were a bit let down by this year's Harrowing thematic, I thought that a good way to introduce a new game-mode in a similar design to Star Guardians Invasion-- as well as keeping the sp00ky theme-- would be a Slayer 2018 event.
Skins:
Slayers:
Shadow Assassin.
Pretty interesting. He seems like the kinda guy to help the team out during a zombie apocalypse.
There's concept art for a Baseball Ekko. Just utilize some of those aspects. Baseball bat with spikes. Lots of spikes.
Zombies:
He is getting a rework soon, so I doubt they'd give him a new skin so soon after. If not him, then Varus.
If you couldn't tell by now, Darkin can easily fit the criteria for Zombies.
Rhaast.
It'd be a PvE event where you as the players could take either side. A Slayer Gamemode where your goal is to survive and escape the city to regroup with other survivors (potentially set in a post-apocalyptic Valoran City??) through different sections (gotta have that obligatory sewer section), and a Zombie Gamemode where your goal is to hunt down all the Slayers throughout the city and turn them. The other Zombie fodder could be Zombie Minions, or perhaps bigger versions of Yorick's ghouls.
In fact, something I just came up with while searching for the following image is-- what if the Zombies are the result of the creatures from SG Invasion taking over? An alternate universe OF an alternate universe! It'd explain the supernatural properties of Ryze and Brand, as well as creating an opportunity to expand Valoran City.
http://img14.deviantart.net/d7c9/i/2009/069/3/1/paradise_city_color_by_neisbeis.jpg
Outside of the gamemode, Rhaast is the Infection trying to overtake Kayn. Killing a ranged champion speeds up the creation of an Antidote, and killing a melee speeds up the Infection, as represented by the Orbs. Upon transformation into Shadow Assassin, Kayn drinks the completed Antidote, getting rid of the infection, and upon transformation into Darkin, Kayn is about to drink the Antidote, but the Infection takes over, making him drop it and becoming Rhaast. In the event that the Zombie Apocalypse is more fantasy-based, by some curse, then Rhaast can be the embodiment of the curse, hence the voice in Kayn's head.
All the Slayers:
Shadow Assassin Kayn Braum Ekko Jinx Pantheon
vs
All the Zombies
Ryze Nunu Brand Varus Rhaast
These would also make for decent teams.
Kayn JG, Ekko Mid, Panth Top, Jinx ADC, and Braum Supp.
Ryze Mid, Nunu JG, Brand Supp, Varus ADC, Rhaast Top.
[slayer-pantheon-thumbs]


or
or
(Smart, trap builders/setter)
or
(Killers, bad ass hunters, 4 horsemen of the apocalypse)