Every so often, HarperCollins puts out slipcased special editions (labelled "deluxe editions") of Tolkien books. 8 times out of 10, these are released the same day as the standard hardback and ebook editions. The other rare times, they come out randomly, or afterwards.

So, which titles haven't been published yet in this deluxe edition format, yet, that possibly COULD?

It would be neat to see the following - this is all 100% pure speculation on my end (anyone from HarperCollins, please take note. I'm not expecting royalties - just consideration!)

The History of the Hobbit

- black and grey book and slipcase (to match The Complete History of Middle-earth deluxe edition from 2017)
- JRRT monogram featured (or, possibly the design of Smaug that's featured on the dustjacket for the hardback) on the book and slipcase
- gold / yellow ribbon-marker
- essentially the same layout as the 2011 hardback edition (incl the frontispiece, and glossy plate section)
(I was thinking of the 2011 History of the Hobbit hardback somewhat matches The History of Middle-earth, in 3 parts w/ the brown, green and blue dustjackets)

The Annotated Hobbit

- green book and slipcase (though possibly orange?? The endpapers from the 2002/2003 editions had orange endpapers...)
- green ribbon-marker
- essentially the same layout as the forth-coming hardback edition (incl the frontispiece, and glossy plate section). They could launch together, like (most) deluxe editions usually do with the standard hardbacks.

The Complete Guide to Middle-earth (illustrated by Ted Nasmith)

- 'Pillars of the Kings' as frontispiece, or possibly another piece as a fold-out sheet
- all of Ted Nasmith's art from illustrated hardback
- text setting from the illustrated hardback
- high-quality glossy paper
- red ribbon-marker
- logo on book and slipcase is JRRT monogram
- book and slipcase is red

The Lord of the Rings Reader's Companion

- same frontispiece as the current edition will be reflected in the deluxe
- Tolkien's alternate design for The Fellowship of the Ring as the motif on the book and slipcase.
- blue or red ribbon marker
- blue or red slipcase

Letters, Author of the Century and Inklings as print-on-demand hardbacks. Possibly The Monsters and the Critics, as well (though possibly, if it wouldn't be too big, the 'On Fairy-Stories' and 'A Secret Vice' sections could be from their own dedicated books)

It's also probably too late now, and because of the size, I'm not sure if they could even BIND it in the deluxe edition format, but The Lay of Atrou and Itroun hasn't been released in deluxe slipcased format yet.

I'm curious what your thoughts are, or if I've missed anything that could be released in the "deluxe edition" line-up.