The thing is, I now don't think such a system is necessary at all. 13th Age is welcome to the escalation die, and may it bring much excitement. But to explain my change of heart I'm going to have to recount the history of boredom in RPG combat. If you want the short version: 1) boredom is not just from high level combat; 2) to fix boredom, use what's already there in the game or piecemeal systems that mean something real.
Our story begins not with the combats of tabletop RPGs, but with the first crude attempts to simulate them, the first so-called computer RPGs like Wizardry and Temple of Apshai. I say "so-called" because there was very little role-playing or sense of wonder about these pursuits. You were running a single-minded band of dungeoneers with no other goal than to map blocky dungeons, stay alive, amass loot and gain levels. Of course, this was not too far off from what the majority of adolescent D&D fans cooked up for themselves around the dining room table.
Combat in a game like Wizardry or Bard's Tale laid down a procedure that with few changes is still followed today in the computer RPG genre, especially the more rules-light, anime-influenced "JRPG" games. You have a lineup of characters; maybe a back rank. When monsters appear they also form into ranks. The figures in turn have a bash at each other, or cast spells, use items and so forth.
The problem with this is that, under standard D&D rules, hit points grow with levels much quicker than damage does. A first level party facing three orcs have to whittle down only 3 HD but each of those does 1d8 of damage, for example. When the party, now sixth level, faces three hill giants, the enemy's HP have gone up by a factor of 8 but damage only by a factor of 2. While first-level characters are only an unlucky blow or pair of blows away from death, higher-level characters don't see combat damage as that kind of immediate threat - it takes multiple rounds or multiple fights to be worn down to the life-or-death point.
You see the tunnel vision? "Combat is a mathematical contest between hit points, armor class and damage. To solve any problem with boring combat we must tweak the mathematical parameters."
Away with that! Combat in an RPG is a tactical simulation. "Grinding" is for machines. Live figures will be running, jumping, diving, bashing, swinging ... And if we see the enemy as mechanical combat, then the answer to boring situations at all levels is to animate it. That way, you don't need to constantly threaten the players with death to get them excited and involved in the fight.
I'll show you what I mean next post, which is about boring combat at low levels.