Slim. It does a decent job of providing flanking opportunities and making tanks more interesting in combat. Adding side armor is possible, but I don't think the benefit would feel that tangible.
This makes me a bit sad. With the current state of heavy armor dominance, and flanking being rather difficult to pull off I would think side armor would add quite a bit more of a middle ground in between completely surrounding an enemy and actually flanking an opponent.
Although I know you guys said you were working on something, I'm curious what you guys at relic think would be more tangible in stabilizing the late game.