Although we do not know what the sealed portion of the golden plates contain, we do know the subject matter.  According to Oliver Cowdery, the first Church Historian, Joseph gave a detailed version of his visit by Moroni:  "The sealed part, said he, contains the same revelation which was given to John upon the isle of Patmos, and when the people of the Lord are prepared, and found worthy, then it will be unfolded unto them."  Being the "same" revelation it will doubltless be an elaboration and clarification of much that is still unrevealed and obscure in the Book of Revelation.

Backman & Perkins, p. 80