the Arbiter, Hunter of those who Fled
Concept: rather than a single named entity, the Arbiter is a title shared between several people or even handed down from one person to another. This allows for more freedom with skin options, and allows the champion to be more like Batman - it's more of a symbol and less of a person.
Mechanical concept: The arbiter harshly punishes enemies that are caught out of position and offers solutions to teams that are fighting against high mobility kits.
Role: Assassin/Mage
Q: Pillars of Law. This is a short range skillshot that impales the target with beams of light, nailing it to the ground for a very short time (fraction of a second). If this ability hits a target mid-dash, the dash is canceled and the target runs back to where it started the dash, (even over walls if necessary) at its dash speed.
W: The Pursuit. This ability passively gives the Arbiter vision of a trail that enemies have recently passed through, and can be activated to move faster while tracking those enemies along that path. It stacks with itself, so enemies that have double-backed will only speed up the pursuit. Flashes, dashes, and wall jumps do not break the pursuit line, though the Arbiter can't cross without a Flash of his/her own.
E: Justice. This ability can't be purchased before The Pursuit. When activated, all enemy targets connected to the Arbiter via a pursuit line are rooted and blinded for 2 seconds.
R: Metaphysical Enforcement. This targeted AOE marks enemies inside the selected area, as well as those in melee range of the Enforcer. For 4 seconds, the mark tracks the movement of those it marks. At the end of the duration, each marked enemy target takes damage equal to the movement tracked.
(also, hehehe, I made an enforcing the meta joke)
Passive: Rule of Law. Any time any champion is rooted, interrupted, or stopped mid-dash (such as E or Q above), the Arbiter gains a permanent, stacking shield. While this shield is active, the Arbiter's base movement speed is increased by 5.
Skin concepts:
base skin - a conglomeration of geometric shapes with face-like images trapped in them. Think something like Zod and friends trapped in the phantom zone at the end of Superman 2. (Was it superman 2? I can't remember) they hover together in a sort of way that indicates a vaguely humanoid figure bound only by magic. The color scheme should match that of the previous Arclight-themed skins for Vel'Koz and Varus. The implication should be that the Arclight characters work with the Arbiter in some fashion.
the Arbiter, Morgana - an alternate universe version of Morgana where she became the Arbiter instead of becoming a fallen angel. She would have a beautiful set of armor and an imposing staff, and that staff's headpiece is one of the pieces of the base arbiter skin, devoid of its face. This would imply that Morgana defeated or destroyed the many faces of the Arbiter and took sole control in that universe. She should be depicted as if she were no longer an official angel, but not "fallen."
the Arbiter, Akali - another alternate universe character, this version of Akali has a less armored appearance, but her armor is still significantly more than a ninja would wear. Her weapon is the staff with a blade attached, the blade is more angled than the one used by her runeterra-prime self. That staff features a headpiece much like arbiter Morgana's. This version of the Arbiter is haunted by ghosts, perhaps those of the other Arbiters.