Actually, this has been debated about quite some times already. I'm too lazy to dig out the threads and links again.
I think most agreed that the Silmarillion could only be filmed in more than one movie. (I - The Rebellion of the Noldor; II - Beren and Lúthien; III - Nirnaeth Arnoediad and Túrin Turambar; IV - Fall of Doriath and Gondolin and the story of Eärendil; V - Akallabêth; - for example).
The story would have to be fleshed out immensely (I,IV and V especially) and would at times be borderline fanfiction. Of course this does not mean bad, but mean straying from the book (which also does not automatically mean bad, though not all here will agree).
The questions that arise are the filming rights and whether such a massive amount of movies would in fact bring profit. But I take we're not talking about that thing called "reality" here.
Maybe some parts of the Silmarillion will be done somewhen, Beren and Lúthien, for example. Or maybe a high-quality animated series. This is cheaper to produce and could stay closer to the book.