tl;dr at the end.
I really enjoy this game, I love the mix of Fighting Game and 3D Action. Played the closed beta and open beta and as I progress I see more and more that a defensive playstile dominates aggressive. Where most fighting games have a rock-paper-scissors mentality behind them, this game has just rock beating paper and scissors. Guardbreaks are easy to break and therefore nearly useless in high level play. So there is nothing that beats blocking or parrying right now. Not only that I really think that being exhausted is not really something of concern, more annoying than dangerous. I would like to propose a few solutions to a few common problems.

To make an aggressive playstile viable again we would have to change the game a bit. In other fighting games overly defensive players can be dealt with a throw. Those are often only breakable within a few frames, so few frames that when you see the animation going it's already too late. This isn't the case in For Honor. Guardbreaks are our "throws" but guardbreaks can be easily countered and punished for. Guardbreaks should behave like throws in other fighting games, very hard to counter and should do as much damage as a heavy attack. This way you would have to read your opponent and act accordingly. Then there are the environmental kills. Those should only happen when you are out of stamina. Besides that you should take more damage and be less mobile when out of stamina so that it becomes a state where you should not be in. This could make some classes like Lawbringer better without buffing the damage or speed. Another thing that should be changed, and this works with stamina, is that parries should drop stamina and not deal damage. This affects both aggressive and defensive players. Spamming attacks till you are nearly out of stamina are a risk and high level plays stop being 3 min. stare downs.

tl;dr
So in conclusion make guardbreak a guaranteed heavy hit with a super narrow time window but no instakill option unless exhausted and make parries deal stamina loss. This would make the rock-paper-scissors approach, that fighting games have, viable again. Attacks get blocked, blocks get guardbroken and guardbreaks get read and attacked or countered. In the end I think this will make the game more enjoyable and ensures longevity.