If you want the game to feel more horizontal, a meaningful midgame milestone around level 40 would go insanely hard - something like OSRS’s Fire Cape: a tough goal with a powerful, rewarding payoff that stays relevant.
No hope needed. As I said at BlizzCon, we'll be adding more content to the 1-60 leveling journey over time, as we add to this horizontally-expanding, evergreen, Classic Azeroth.