"Gandalf and Thorin arguing [from The Quest for Erebor] once Bilbo was carted off from his fit...."
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I like that concept as a way to bridge the children's Hobbit and adult LOTR.
I rather doubt PJ has read works like UT (especially footnotes, too much work

)
but I think there are hints that P. Boyens and/or Fran Walsh have. It could
work as a prologue, especially to get non-book attendees up to speed on The
Hobbit and its connection to LOTR.
Suggestions like Ian H's of a tv limited series would work well but I think there's
just too much $$$$$ in movies (and more scope for PJ's pyrotechnics, which is
both good and bad- since he seems to get carried away with battle scenes).
Still, the Battle of Five Armies is almost tailor-made for him, especially with the
big finale with Beorn.