In my experience playing the game the first thing that stood out to me was how unbalanced the game can be. As far as pvp is concerned there is no question that the nobushi and the peacekeeper are game breakers that just destroy the balance of combat. Starting with the nobushi, she has the a complete advantage as far as reach is concerned, especially with her guard break. This is how I'd fix her:

1. make her moves actually counterable, slower charactes (anyone from the viking faction) despite nailing the counter will just end up missing the attack and getting massively punished for it. Considering that this game in an attempt to keep things realistic while balanced, needs to have this feature, just hypothetically as a viking warlord who just countered the nobushi's attack it doesn't make sense to just hit the empty air, you would do a quick jump or step forward to make the move connect.

2. remove bleed or give other characters as debilitating (any stun the vikings or knights do just doesnt have much of an effect, you can usually only get in one light attack) of a debuff, playing as the warlord it only took three bleed attacks and that was game over, the other person would just run away til you bled out. Them being faster means you're screwed.

peacekeeper: refer above about bleed except that she only needs two of the shank moves to kill someone. she is plenty fast to be a real threat without bleed.

Due to what I've explained above this is why people only go for the peacekeeper and nobushi. without being them the only other option is to be the raider and hope you can successfully throw them off a cliff before you bleed out.

despite these flaws I love the game, it is a blast to play, and amazing both with the graphics and the gameplay. While I know people will always gravitate towards things that make the game easier or at least make them win more, I would like to remind everyone that not everyone has time get amazingly good at all the characters, so making each character able to hold their own would make the game more immersive and attractive to play, I mean why play a game about samurai vikings and knight if everyone is only ever the peacekeeper or nobushi?