Playing devil's advocate, seems to me having a story based on lore in the Appendices is the safest bet as far as adaptation goes. You have the outline for framework but within that lots of room to maneuver. Unlike adapting a fully fleshed story such as LoTR or TH I think it'll have a much better chance at being good, since there's less screen to book comparisons.
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Morsul the Resurrected
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