If I were to replace any of them, I would say Drapion.
We all love Drapion, and it's somewhat heartbreaking for me to say this, but he isn't doing much. Zoroark has the dark side covered, and Poison type moves are really not useful in BW2. From my experience with the game, I would say Drapion is good for a buffer Pokemon (as in when you just want a sixth Pokemon to complete an already set team), but if there is a better alternative, take it.
Also, Walrein is really good in Skyla's gym and in due course, Drayden's gym. Not that your team really needs Walrein to deal with them, but it's good to have back up.
Hope I helped!