Champion Concept: Avatar
For a TL;DR, just look at the image linked below
Preface
Most champion concepts put forward by players are very thorough. They have all four abilities and a passive drawn up, they have some kind of character description with lore, they might even have a pretty drawing. None of them ever get implemented. The closest we've ever come is maybe Braum, who was probably inspired at least a little bit by the "Angus" comic strip.
I'm not going to do any of that. I just have an idea for 1 core game mechanic. It's pretty different from what's been done before, I didn't draw up a kit because this mechanic would require a lot of playtesting before it becomes clear what his/her kit needs to function. I'd rather leave that to the professionals, because I'm probably not gonna come up with a kit anywhere near as good as what they can make.
Full Explanation
This was my idea after trying to design a bot lane melee adc. There are a lot of problems inherent to melees that prevent them from substituting marksmen. First, melees generally cannot "trade" with ranged champions. What usually happens is the ranged champion just pokes them down and zones the melee, until the melee reaches a powerspike where they can zone in return using raw kill pressure. The threat that they might jump on the ranged champion for an all-in, and win with their higher base stats.
Yasuo sort of solves these problems. He is playable mid and bot lane. His kit allows him to negate projectiles, withstand some poke, and fight at melee range almost whenever he wants to. He's designed to remove the ranged advantage. Fizz and Riven can do the same with their mobility and unique defenses, but to a lesser degree.
The only point where Yasuo and all other melees seriously struggle is sieging. They cannot waveclear, or attack turrets, without putting themselves in a seriously dangerous position. So it becomes very hard to press a lead, even worse if you're behind and can't zone enemies off your turrets with kill pressure like I described above.
The third problem with melee carries in general (and by this, I'm referring to every glass cannon melee in the game) is teamfighting. Marksmen can survive clustered fights by standing on the outskirts, taking little or no damage. Melees don't have that option. So to survive, they need some combination of higher tankiness (but can't heavily itemize for it because then they're not carries), and specialized defenses. All glass cannon fighters have some solution to this problem. Master Yi has slow immunity, short periods of untargetability, and strong damage reduction while meditating. Tryndamere has undying rage. Yasuo has Wind Wall, his shield passive, and can use his E to dodge some abilities. Fizz has his passive (which is mostly just for laning), and the trollpole. Fiora has Blade Waltz. They also all have gapclosers so they can't be kited or peeled easily.
So what does this champion do?
This champion has an untargetable spirit that the player can control. The design is similar to Azir, but with several differences. First is that the Ghost is not controlled with basic attacks. When the Host is in the meditating stance, you lose control of them and control the Ghost instead. The Ghost can use basic attacks and abilities like a normal champion... but it cannot be attacked directly.
The Host stays on the map. That's to balance out the player controlling what is effectively a permanently untargetable champion. The Host cannot move by itself. Maybe the Ghost can move it in some way, or at least peel for its Host.
The skillset would be a bit like Gnar's. Mini Gnar is a very weak champion by himself, with a very limited set of abilities and poor base stats. The Host would be similarly weak. The Ghost has a different set of abilities, likely including strong mobility skills and steroids. Maybe also some utility to protect its Host, either by moving them or cc'ing enemies. Azir has very powerful self-peeling tools to compensate for his awkward kiting, this champion may require tools of similar impact.
The Host and Ghost are always connected. The Ghost cannot travel outside a limited range. It can't be a long tether range, because the Ghost would then be able to attack turrets without needing someone to tank the aggro. CC probably carries over; because the two are synchronized, stunning the Host would stun the Ghost as well.
That's the basic concept. An AD reverse Azir where you play as the soldier.