I think the only real complaint I have for the three films actually being a triology is that there is no unifying story arc. At least not as shown.
Ok, that is a terrible sounding paragraph. But I'll leave it there as a weak attempt at what I'm trying to say. He goes try number two.
While FotR, TTT, and RotK all can stand alone, with a biggining, middle and end; the quest of the ring links all three films. The stories from the Silmarillian are even more independent than the three volumes of Lord of the Rings. But still have that singular overarcing quest.
The Silmarils.
We all seem to have strayed from a prime focal point. Suddenly with this the focus of any "Silmarillian" films can change drastically.
Beren and Luthien would still be a prime story with direct influence on the Silmarils.
This would also mean that the voyage of Earendil and the war of Wrath have to be added to the mix. That really is the conclusion of the Silmaril arc. And really, the reason the Silmarils cause so much anguish is the Oath of Feanor that drives his sons to stike down anyone who would take and keep a Silmaril from their rightful owners, themselves.
Greed and the love of things is what drives the entire overarcing story. At least that's how I read it.
And then, with this as the overall story arc, it's not quite as much of a tradgedy story. Because in the end Melkor is overthrown, the Elves are welcomed back to Valinor, and some have proven the potential of men.
