As far as I know the only 'knights' in the Diablo universe are the knights/soldiers of Westmarch and the paladins of Zakarum. According to your description the paladins are the knights you seek. Since we know that the barbarian is the only returning character the paladin won't be in it. I do want to say that I'm no lore buff, and Blizzard has a lot of freedom in creating his characters, so this point might be moot.
But think of it from a gameplay standpoint. How much different would a knight be from the current barbarian and monk? The monk is a mix of melee damage and holy magic. The barbarian is a heavy melee fighter. The knight would have an aura/buff system for the party, but what would he do on his own? Blizzard stated that each character is self-sufficient, having a leader without a party is not very useful. If you take away the party a knight would be the same as barbarian.
Also, the currently is no class that is proficient with bows. I think a bow-wielding class makes much more sense than a party leader, since wielding a bow is a large component of D&D.