They said that these questlines will be retrospectives that explain what given race is about. Let's speculate what stories will we see.
Human - Gnoll war/First war to deepen faction conflict
Night elves - recycled War of the Ancients to build up Azshara more/ banishing of Quel'dorei for more faction hate
Gnomes - fall of Gnomeregan
Dwarves - War of Three Hammers/something that happened between W3 and WoW, some Forsaken lore
Draenei - Fall of Shattrath
Worgen - civil war with Crowley?/something from the past?/starting zone again?
Pandaren - war with Mogu, reminding of unity to pandaren/Liu Lang story/Burdens of Shaohao
Orc - Exodus of the Horde
Trolls - Gurubashi civil war
Undead - happenings before WoW to see why they hate Stormwind/Fall of Lordaeron, Arthas hype, showing Forsaken as poor victims
Tauren - some past events?/war with centaur?
Blood elves - they said that we will see Arthas' attack on Quel'thalas
Goblin - Kezan's golden century/retconned Cataclysm with azerite/war with Zandalari
Pandaren - war with Mogu, reminding of unity to pandaren/Liu Lang story/Burdens of Shaohao