"Stealth" concept

This is an idea for a stealth-like champion kit that doesn't actually mess with vision, but still aims to go "surprise, enemy here!" via deception.

The underlying theme is body-swapping/face-stealing/etc. This should give you a good idea of where I'm going with this.

The core ability would be a short-ish range gap closer. This ability switches the appearance of the champion and target - that is, on screen and minimap, the target now appears as an enemy champion, and the user now appears as an ally (to the enemy team only). I'm not sure how deep this should go. At a minimum, items should display as the other persons, and maybe health. Actual stats would be unchanged - a keen eye may notice someone moving too slow for what their items suggest.

Now, obviously, being able to look like you're on the other team (and have them look like they're on yours) lets you get away with a lot - this obviously breaks on attack (for you only - no breaking it with a quick attack on a creep for the victim, they're stuck with your face). So you can jump the person in mid from jungle with this... and then saunter bottom looking like them while they're stuck with your face. If the enemy team isn't paying attention, they may try (and fail, this doesn't allow friendly fire... probably.) to gank their own teammate while their bottom lane moves in aggressively expecting a gank... only to find it's an enemy nomming them from behind.

The duration of the effect would be around 30 seconds I think for the face switch, enough time to get somewhere and use it.

The obvious requirements to complete the deception are that the user broadcasts their vision to the enemy team, and the victim to the allied team (the former because not doing so would be obvious, the latter for symmetry and to keep track of any mistakes it causes).

The counterplay is that true vision breaks the illusion (the effect isn't removed, but moving within enemy/allied true vision makes you appear correctly, so a laner caught in this can walk under turret and suddenly there's two of them on the map and something's very wrong).

Now, to scale it up for an ultimate we go to full on body-swapping. Or really, body-snatching. The champion would jump into an enemy, dealing a decent chunk of damage in the process. For a short duration (3-4 sec?) they would be in control of that champion. Still no friendly fire from enemy team, because if the taken champion would die in the process... well they die, but the body-snatcher lives on inside their skin, in a sort of permanent version of the face-swap (in part - they appear friendly to the enemy team, but there's no minimap games going on, so it's much easier to tell). This effect replaces abilities completely, and cooldowns are whatever they were when you connect (so if they blow their ult, you won't be able to use it against their team). The body snatch lasts until you die, or until you successfully snatch another body (or maybe until you attempt to). In order to allow use of both AD and AP based targets, your bonus stats are somewhat rebalanced during this: AP and AD are each equal to a weighted average of the two (as AP is about half the cost, they each would be (2*AD+AP)/3 or something, and then a bonus on top of that scaling with ult rank, like 10/20/30% or so). Armor and MR are averaged (again, bonuses only). Attack speed... I don't know. Probably that and CDR unaffected as both AD and AP items can bring these fine.

So basically, you turn into them, with the right stats to make people suffer (and potentially REALLY suffer, depending), and have their kit, and are more gold efficient due to the bonus power from the ult, and in a fight they're going to have a hard time aiming (unlike the basic ability, this doesn't break when you attack, but you may flash red on the team colors for a brief bit, or just be targettable, or some other UI clue).

This obviously isn't totally fleshed out, and may be too aggravating to be really useful. The ult might not confer total control until after they're killed and instead be a debuff representing fighting for control, and then it could last longer (like Morde ult almost). Of course you'd be untargettable for it, a big advantage, and the possession may sort of heal you (you might gain a percent of their max HP in bonus stats for instance).