Since the new Runes are still in early development: The keystone rune I'd like to see

TL;DR Rune that exposes one enemy champion that you stun, making your team's attacks deal bonus damage to them.

In the current mastery system, there is no keystone tailored for a 'catcher' playstyle. Champion's like Blitzcrank and Thresh will usually go Courage of the Colossus, which works for them but just doesn't really compliment what they like to do.

My suggestion is implementing a keystone rune in the new system that works something like this:

Your first movement disabling ability will additionally expose the target for 4 seconds (only works on one enemy). The first attack of each individual ally champion will deal an additional 8% of the target's current health as true damage but no more than 3 times. 90 seconds cooldown, ticking down twice as fast when no enemy champion was damaged in the last 5 seconds.

Let's quickly go through it: The rune would be targeted at a very specific playstyle: Catching out a single target and killing it together with allies. Current health percentage based true damage is very specific in not being able to ramp up too much, but always being relevant. The cool thing about the rune would be that champions like mid lane Lux or top lane Shen could still opt into it while giving up on survivability or damage for themselves. At the same time champions like Thresh or Leona would get a way to meaningfully contribute to their team's damage onto tanky enemies while they can currently only CC them and hope for allies to have enough damage to kill them.

Hence it is

1. Strategically unique
2. Easy to understand (group with allies and CC someone)
3. Supports a specific playstyle, but
4. potentially works on many champions
5. Has a clear works- well- against- pattern (good for dealing with juggernauts, not so good vs protect- the- carry- comps)