Campaign is still heavily in development, there are 3 chapters missing and prologue recently got entirely reworked, though in recent months they mainly worked on the last three specialties (Falconer, Runemaster and Warlock and managed to get them on release and not even very bugged, though notably runemaster is imbalanced because it's the only wizard specialty where you don't have to shot your own foot by mandatory taking the godawful Focus skill, but that's more of other wizard specs being lackluster rather than RM being OP).

What alternate paths? Campaign is super linear, except you can skip some of it by going into dungeons. Also, it's still way better campaign than in PoE, though i wish there would be option to skip it entirely (I play this since early beta and i really seen it too many times), it's one of the best features in D4.