[Suzerain of Sheol]

Um... well, Unfinished Tales is a very strange book. It's from extremely early in his writing and is as much a story about his process of developing the setting as it is about the setting itself. The stories in it are all weird variants and precursors to stories in the Silm.

The Book of Lost Tales is a bit better, being the sort of deleted scenes to his main books, but even then, they weren't finished. Christopher made the best with what he had. I specifically remember "Of Tuor and his Coming to Gondolin" just ends abruptly halfway through, which is a shame. Tuor was awesome, from the little that's written about him.

The thing about the Fall of Gondolin is that it's a rather long, mostly complete story arc, spanning several chapters spaced out throughout the Silm. and given more detail in Lost Tales.

Beren and Luthien is another longer story, and I think there was more to it that didn't make it into the Silm.

And the Children of Hurin was one of Tolkien's favorites. He wrote many different versions of it, including a multi-thousand line poem in perfect verse and meter.

I can't think of any other tales that would work as stand-alone novels. The only other story of that length that comes to mind is the Akallabeth, that is, the Downfall of Numenor, but unfortunately, that story takes place over hundreds and thousands of years and only really works as a history-book type recounting. There are no consistent focal character (besides Sauron. :p)

I do wish that more had been written about the War of Wrath, as it's depressingly under-detailed and sparse for being the most epic battle in the history of Arda.