I think it would be nice to have a landmass between the Veiled and Forbidding Seas. No one ever returns when heading that direction, perhaps its not hidden but rather protected.
Perhaps it could be where the Faceless Ones come from, a race that precedes the titans arrival to Azeroth, and maybe even the cause of the Old God's being summoned to Azeroth through the power of their ancient empires. (this parts my own fanfic, I made a whole expansion idea sheet on this continent once but its been deleted along with my old computer)
Or maybe.... Nothing is there but ancient sea monsters that we will never see.