Only seen twice, once in the beginning and once at the end could mean its on the front cover and at the end of the book. So that means its Eshonai, except she is in WoR and she appears twice in the WoK if I recall correctly.
The other assumption is that it is the shardblade, Oathbringer. This is seen sometimes when read, but you would not be reffering to its locations in the book, except that you must because it only appears on the front cover, not the back.
The next is the chasms. These are on both covers. Many die if they ever seen the bottoms, or only the dead see them, besides some bridge crews. Unlikely.
The last you have already said is incorrect: The storm, either the Everstorm, which it isn't, or a highstorm.
By pure logic, my answer must be that the thing is that greatshell on Dalinar's banner!
EDIT: Wait, is it the Blackthorn version of Dalinar?