Strider's story is not that wide open. We know he has to live and that is fostered by Elrond and falls in love with Arwen. We know he rode with the Rohan and fought for Gondor and spent decades as chief of the Dunedaina Rangers fighting orcs and other unsavory things in the north. We know he befriends Gandalf and fights to protect the Shire and the last of his people in the North.

If it isn't him and Gandalf then who would it be? Most of the main characters in the Silmarillion are unknown to the average fan and almost all of the major ones are either elves or gods and their stories span thousands of years. If you want to a story in the Middle Earth that is easily understandable to average fan then the time between The Hobbit and The Fellowship is the obvious, in my opinion at least, choice. Think like a producer looking to make millions of dollars not a fan wanting to see a living version of a book you love. The bulk of the hardcore fans will watch it even if it sucks and all their bitching is free social media press.