When you have completed your third zone, you should be nearly max level, but you still have work to do! Head back to your faction hub and complete the last foothold quest to unlock your fourth mission table champion. After, you will want to do your newly available invasion missions for your faction. This quest chain is one that will be time gated by reputation levels earned with the 7th Legion for the Alliance and iThe Honorbound for the Horde. Complete as many quests as you can and then finish up your leveling with side quests you may have left in previous zones as necessary.
Baelgun - Alliance Baelgun - Horde Balnazzar - Alliance Balnazzar - Horde Blackhand - Alliance Blackhand - Horde Blackmoore - Alliance Blackmoore - Horde Blackrock - Alliance Blackrock - Horde Blackscar - Alliance Blackscar - Horde Bloodfeather - Alliance Bloodfeather - Horde Bloodhoof - Alliance Bloodhoof - Horde Bloodscalp - Alliance Bloodscalp - Horde Blutkessel - Alliance Blutkessel - Horde Boulderfist - Alliance Boulderfist - Horde Bronze Dragonflight - Al... Bronze Dragonflight - Horde Bronzebeard - Alliance Bronzebeard - Horde Burning Blade - Alliance Burning Blade - Horde Burning Legion - Alliance Burning Legion - Horde Burning Steppes - Alliance Burning Steppes - Horde Bladefist - Alliance Bladefist - Horde Blade's Edge - Alliance Blade's Edge - Horde Booty Bay - Alliance Booty Bay - Horde Borean Tundra - Alliance Borean Tundra - Horde
From my understanding, the world boss is always there when your faction controls the Warfront. Like other world bosses you can only get gear from him once a week. There are other world bosses in the zones outside the warfront and 1 spawns a week just like Legion. I’m not 100% sure that the warfront will be available on launch but I have yet to read anything that says otherwise. The quest also reset once a week as well. Think of Warfronts like old Tol-Barad