Champion Concept: A theater-themed champion
Not gonna start off with some lore, name or whatever as this concept is more focussed on the general theme and the gameplay connected with it.
When I went through our quite large champion pool I had to notice that we have not a single champion with an acting-theme (or general a cultural theme apart from 
), so I thought this will be a chance for a "Phantom of the Opera" like fellow to take his place in the game. The main inspiration for this was the disguise-as-teammate item I have seen on the PBE that will be useable in the Black Market Gamemode.
Before getting to the abilities and gameplay aspects we should get the theme laid out: what is an actor? And what does it mean to "act"? And how does is this useable in a Moba?
If someone has ever been backstage to an opera house or an theater, the first thing you notice is that it's a freaking labyrinth. I'm working in that area for some years now and even I happen to get lost regulary. While the stage from the outside seems to be a very simple thing, the stuff invisible to the audience is an extremly complex cosmos of props, costumes, storage rooms, tech hubs and practice rooms mashed together.
So, our champion dwells in a complex and confusing envoiroment where people could ge tlost and only comes out to act his part sometimes? Sounds pretty much like an Assassin/Jungler to me! I know hear some parts of boards cry out "WE HATE ASSASSINS! SCREW THEM! NO MORE!" - but that's just because several asassins in the game right now have massive counterplay issues or simply overtuned damage-values.
Buuut we are getting a bit too gameplay technical again, let's focus on the theme for now:
First, I'd love to see a champion like that being mysterious, the face always hidden by a mask, so not even the player knows who exactly he is playing right now. The champion could fake the voice all the time, sometimes speaking in a male, sometimes in a female and sometimes in a fake-monster-way (think of a bad Cho'Gath impersonation ^^).
Also, when the champion dies, he (or she, who knows? ^^) performs an overly dramatic death and vanishes into a trapdoor right after when no one is looking. That's a neat death animation for that theme I guess...
About the weapons used: I'm thinking of daggers, short blades and trickery items such as smoke-bombs, or maybe an hextech-powered pyrotech, you know, like the stuff they used in theater sometimes for "special effects" (good thing there are safety and fire regulations today, but back in the 1700-1900 there were cases of theaters burning down due to pyrotech gone wrong...). An essential part of this champions kit should be the ability to disguise...
Okay, now, let's try to turn a theme like that into a working kit. Not going to bother with basestats as that is something for playtesters to figure out, but I thought of this as some kind of mellee, on-hit AP champion, maybe similiar to Diana in that regard.
Passive: Encore: After killing a unit the actor reaches a state of euphoria, getting increased attack & movement speed as well as bonus-on-hit damage. This bonus scales on the enemy killed (champions give much, minions not so much, jungle mobs somewhat).
Q: Masquerade: Can be used to take on the form of a nearby unit, be it friendly or hostile. This skill can be used on Champions and Minions. After a short chanell the actor changes his appearance into said unit. If used on terrain the actor becomes stealthed until moving/attacking or taking another action, hiding on the wall behind some kind of fake-wall (detectable by pink-wards and red trinket sweep).
When disguised as a minion basic attacks DO NOT BREAK the spell, but only deal minion-level damage. Reactivating Q or using another ability (or moving in case of terrain use), will cause the next damaging ability or attack to apply bonus magic damage for a short time. Attacking allied units while disguises as an enemy minion will not cause any damage.
W: Smokebomb: Drops a smokebomb in a nearby area that slightly damages enemies and blocks their vision. Enemies inside no longer share their vision with their teammates and the enemy team cannot look inside for the duration. The actor's movement speed is massively increased upon cast, but decays. The channel-time of Masquereade is reduced while inside the smoke.
E: Chained Daggers: Throws a dagger with a chain attached, when hitting a target the ability can be reactivated to sligthly pull the target a bit closer and apply a minimal slow. Can be recast when missing, but costs extra mana with every try. Mana costs back to normal once the ability goes on cooldown.
R: GRAND FINALE: Throws a projectile on low range that turns into a giant and overly flashy fireball (with pretty fireworks attached to it). Deals primarely Missing-% damage and is supposed to work as an overkill-looking finisher. The base damage of this ability is supposed to be extremly low, as it's not a tool usable for trading. Enemies directly hit by the explosion lose their vision for a moment.
My theory behind this kit is to have a strong, but fragile asassin that has to use his non-damaging tools Q and W to get close to unsuspecting, low health targets, getting them with the element of surprise and if that doesn't work you still have have that one dirty trick with your E ready. The ultimate serves two purposes, depending on how it is used: using the vision-block as an escape if smokebomb is on cooldown or that's simply not enough, or using the %health damage of it as a finisher. As the ability will be on a fairly high cooldown, you should be careful what to use it for...
Also, the passive is there to encourage a risky playstyle: if you manage to kill an enemy champion, you have most likely used all your tools, especially your ult. But on the other hand you are just buffed up... so, do you try to go for another kill or not? ^^
I wonder how you like my idea and I will modify this concept according to the feedback I'm getting...
Hoep you like it :D