The classic flavor to me isn’t the fact that horde doesn’t have paladins and alliance doesn’t have shamans. What makes vanilla different is stats aren’t inflated. TBC actually wasn’t too far off stats wise from vanilla, just a little bump, but it took off in wrath and took off the most in Cata, where you get the faceroll and anti-social effects.
Also being pidgeon-holed into content areas such as Outland in TBC and Northrend in WotLK just made the world smaller. Additional vanilla content could be just a new zone or… maybe not even a new zone, maybe expand upon the existing zones and add dungeons/raids/quest lines there.