I would go for:
Mage - The Nexus
Hunter - Amberpine Lodge or Silverbrook in the Grizzly Hills
Warrior - Amphitheatre of Anguish in Zul'drak
Priest - Onslaught Harbour as @
Varodoc suggested is a good one, alternatively New Hearthglen
Warlock - This one is tricky. Dreadscar Rift wasn't great in my opinion. I can't think of anywhere in Northrend that would feel fitting for them. Possibly Black Temple? But that has no link to Northrend so I'm stumped for Warlocks.
Rogue - The pirate cove in Howling Fjord
Evoker - Wyrmrest Temple
Shaman - River's Heart in Sholazar Basin
Druid - Cystalsong Forest
Death Knights - Archerus again but over Icecrown
Paladin - Lights Hope Chapel was ideal and has obvious links to Northrend in terms of a throwback to Wrath.
Just like Warlocks, I'm stumped for Monks and Demon Hunters if we're going off a Northrend theme.