My ideas copy-pasted from the previous thread mentioned above:
Something interesting or quirky or unexpected, like the comic book in TESA: Redguard's manual.
A piece of literature like the Pocket Guide to the Empire, but one that better resembles what the PGEv1 represents: in-depth notations of conflicting ideals and philosophies within the world.
A map, naturally. Hopefully, one a bit more useful than the one that shipped with Oblivion. And, though it was pretty damn cool, perhaps one not so revealing as the one that shipped with Morrowind.
A phial of skooma.
A soul-gem.
On the subject of the map, it would be nice to see a product that didn't use high-gloss paper. Redguard's map was printed on stockier material, and Todd Howard came up with the rather ingenious idea to go out and blowtorch the edges a bit, giving the map a nice burnt feel. Perhaps the map could be done on some form of cloth, if only for a different approach.
More ideas:
A miniature Heart of Lorkhan, complete with miniature wraithguard, keening, and sunder.
A miniature figurine of the Numidium.
A Dwemer Puzzle Box (with an actual built-in puzzle to solve happy.gif)
A weathered and worn-looking banner with the Imperial Dragon symbol on it. [Might not apply with recent information regarding the ES novels]
A weathered and worn-looking "Wanted: Gray Fox" poster. [Might similarly not apply with the time-gap anticipated between TES:IV and TES:V]