Bruisers could use a rune that capitalizes on both their damage and their durability. This idea would replace Overheal's concept to something that benefits from a balance of damage and defense, but can be used to some effect with more damage than defense and vice versa.
> Gain shield stacks of 2% (+1% per 300 bonus health ) of the damage you deal plus 1% (+1% per 100 AD/AP) of your maximum health every unique champion hit per attack or cast, up to a total of 5% (+2% per 100 bonus health) of your maximum health. Shield stacks decay after 1 second1 if not depleted. Only 25% effective vs minions and monsters.
A shield stack is an individual part of your shield from this rune, and each stack has an independent timer. Shield decay rate is debatable, but my suggestion is that a full stack would decay over 1 second at a constant rate, while partially depleted stacks will have a proportionally faster decay at the same flat rate as if it were a full stack. If you hit the total cap, the oldest stacks are depleted and replaced by the newest stack. The on-damage behaves similarly to Electrocute stacks. The AD/AP scaling is combined: 50 AD and 50 AP will give +1%.
Having a lot of bonus health increase the X% damage increase and cap, and a lot of AD increases the Y% maximum health gain. Glass cannons can't use this effectively because both parts scale with bonus HP. Pure tanks can't use it effectively because the %HP gain doesn't scale well if they lack bonus AD or AP.
The numbers may need tweaked, but the idea that since both parts scale with AD/AP and HP, it means that most Juggernauts and Vanguards will use this very well.