All armour is specific to classes anyway, so why aren't there class specific armour exotics?
I'd make class exotics lore specific. They would be difficult as fek to earn. Titans: The actual mark of Saint-14, earned by a long quest that leads back to the tomb. Warlocks: Toland the Shattered' bond, earned by a quest that takes you back to Luna to find Tolands body. Hunters: Tallulah Fairwind/ Andal Brask/ Cayde-6's cloak (not that knock-off eververse one), earned by talking to Shiro-4 and Amanda, they tell you to go a quest in the cosmodrome and visit Cayde's grave.