If Tolkien wrote one more book what would you want it to be about?

[The Squatter of Amon Rûdh]

Here are a few that spring to mind

Old English for Dummies
Beowulf: Original and translation on facing pages, with introduction, notes and appendices (I'm sure he'd have some more revolutionary perspectives to offer by now).
Poetry of the Eldar - New Pieces with Commentary
On Translating Sindarin

I wouldn't mind seeing some quietly satirical opinion pieces as well. They could be about things like:
Television
Motor cars
Modern English Literature degree syllabi
The New International English dictionary
Dame Barbara Cartland
The plan to mine for coal beneath Lord Byron's home
etc, etc, ad nauseam...

Anything where his sense of humour could come across anyway. I'd like that.