Updated to 3.6 (April 2016) with expected 3.7 material. As this is a content timeline of STORY, this is going in the LORE section; it will not be including Intrepid anything, unless it helps story progression (like Intrepid IA: Hammerknell).
Will not differentiate between Normal and Expert mode dungeons (or the few Master modes, at that). Just do the Experts when you're done with a game version, when you're directed to it by a questline, or whenever you feel like it.
G = Guardian only
D = Defiant only
Events = Misc. themed zone events, invasions, rifts, instant adventures etc
Bolded = Open Zones
Italicized = Instances (Chronicles, Slivers, Raids, etc)
Underlined = Epic Quests
Note: Aelfwar/Life Saga, Endless/Death Saga, Abyssal/Water Saga, Wanton Maw/Fire Saga, Storm Queen/Air Saga, Lord Arak/Nightmare Saga; and quests may weave around other content, so advance as best you can in a saga before moving on, entering dungeons and zones
CLASSIC (Lv 1-50), 2011-2012:
-Mathosia (G)/Terminus (D) starting zones
-Silverwood (G)/Freemarch (D) zone quests and events, with dungeons Realm of the Fae (G)/Iron Tomb (D), with the start of Life Saga (G)/Death Saga (D), which continue through Classic zones
-Gloamwood (G)/Stonefield (D) zone quests and events, with dungeons Darkening Deeps (G) and Deepstrike Mines (D)
Fork Option 1A, West:
-Scarlet Gorge zone quests and events, with Foul Cascade dungeon
-Scarwood Reach zone quests and events, with King's Breach dungeon
Fork Option 1B, North:
-Iron Pine Peak zone quests and events, with Abyssal Precipice dungeon (higher level); Air Saga begins here and continues through Classic and Storm Legion zones
Fork Option 2A, South:
-Droughtlands zone quests and events, with Fall of Lantern Hook dungeon
-Shimmersand zone quests and events, with Charmer's Caldera dungeon
Fork Option 2B, North:
-Moonshade Highlands zone quests and events, with Runic Descent dungeon
-Stillmoor zone quests and events, with end of Life Saga (G)/Death Saga (D)
-Mathosia (region) expert and raid rifts
-Greenscale's Blight chronicle and raid, River of Souls chronicle and raid, Gilded Prophecy and Drowned Halls slivers
-Sanctum (G)/Meridian (D) Ceremony of Attunement chronicle
-Hammerknell chronicle, IA (Lv 65 version of raid available), Water Saga, Rise of the Phoenix sliver
-Ember Isle zone quests and events and raid rifts, with Caduceus Rise dungeon, Fire Saga, Infernal Dawn raid, Primeval Feast sliver
STORM LEGION (Lv 50-60), 2012-2014:
-Air Saga continues, Exodus of the Storm Queen dungeon
Fork Option 1, Brevane:
-Cape Jule, City Core, Eastern Holdings zones quests and events, with Storm Breaker Protocol dungeon
-Ardent Domain, Kingsward zones quests and events, with Golem Foundry and Empyrean Core dungeons
-Ashora zone quests and events (particularly Volan)
Fork Option 2, Dusken:
-Kingdom of Pelladane zone quests and events
-Seratos zone quests and events, with Unhallowed Boneforge dungeon
-Morban zone quests and events, with Archive of Flesh and Tower of the Shattered dungeons
-Steppes of Infinity zone quests and events
-Brevane and Dusken raid rifts, Hunt and Great Hunt rifts and their questline, Triumph of the Dragon Queen sliver
-A Hero Rises chronicle and Frozen Tempest raid at the same time, Endless Eclipse raid
-Dendrome zone quests and events, Hive Kaaz'Gfuu chronicle, Infinity Gate and Planebreaker Bastion raids, Planebreaker Bastion chronicle, Grim Awakening sliver
-Infernal Dawn chronicle, Realm of Twisted Dreams and Return to Deepstrike dungeons, Ember Isle Breaker Point quests
-Moonshade Highlands Runic Athenaeum quests, Bindings of Blood raids
-Bloodfire Army invasion events, with end of Air Saga
NIGHTMARE TIDE (Lv 60-65), 2014-2016?:
-Nightmare Coast quests and dungeon
-Goboro Reef zone quests and events, with Gyel Fortress dungeon
-Draumheim zone quests and events, with Citadel of Insanity dungeon
-Tarken Glacier zone quests and events, with Glacial Maw dungeon
-Return to Iron Tomb and Return to Empyrean Core dungeons
-Plane of Water raid rifts, nightmare rifts
-Shadow From the Beyond chronicle, Rhen of Fate chronicle and sliver, Mount Sharax raid
-Tyrant's Throne zone quests and events, Tyrant's Forge raid
-Nightmare Saga
-Planetouched Wilds zone quests and events, Rhaza'de Canyons dungeon
-Mind of Madness IA and raid, Comet of Ahnket sliver
-Possibly more
4.0? (Lv 65-?), 201?
-?
I may be incorrect or too vague, particularly for Nightmare Tide content, so feel free to correct.