Idea about Shield Bash for tanks

Shield Bash

After damaging enemy champion with an ability will grands you a shield that decays over 5 seconds shield amount: 30 + 15% bonus health cooldown: 40 – 20 seconds (based on champion level)

This kind of rune will benefit laning tanks who able to trade with their opponents, aggressive melee supports such as Braum and Leona that wish to start a bot lane skirmish while reducing their risk of as a melee support and junglers when they gank a laning opponent or a skirmisher between jungler.

Lategame Teamfight can last somewhere between 5 and 10 seconds so 5 seconds decaying shield would be enough as tanks usually gets avoided so their shield lasts longer. 15% health scaling encourages heavy durability builds from items that provide lots of health. Longer cooldown at early game will push tanky champion for heavier damage trade with their opponents for longer period of time rather than short harassment (aka cheesy attacks) while shorter cooldown for late game since teamfights happen more often at this point.