When we were talking last week about things that make your Garrison feel like home, one thing that cropped up was ways to show off achievements past. Mumper mentions that there will be Monuments that players can place, earned from achievements. However he does go on to clarify that Garrisons are not about placing furniture and decorating. This isn't The Sims in WoW, after all.
Most exciting, though, he talks about one building, a Mage Tower spec called City Portals that opens ports to every major city. It seems from his later tweets that the tower will not be mage-specific, but will need to be specced and upgraded like any other building to get the portal setup. His response to another tweet confirms that the plans will be from drops, quest rewards and vendors, again implying the buildings will be available to all.